Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a bat coronavirus that causes coronavirus disease 2019 (COVID-19). The virus emerged in late 2019. By January 2020, scientists had isolated the virus and defined its genetic sequence. With that information, they could work to make a vaccine to protect people against this disease. 

COVID-19 vaccines became available in the U.S. and other countries by the end of 2020. They targeted the spike protein of the virus. The spike protein is how the virus attaches to cells. COVID-19 vaccines were made so quickly because massive financial and human resources were dedicated to the goal. This included the participation of tens of thousands of people in the clinical trials. 

The results of the clinical studies were carefully reviewed to ensure the safety of the new vaccines. In the U.S., this review was done by independent experts advising both the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC). Importantly, COVID-19 vaccine development was not done quickly by skipping steps, but rather by conducting several aspects of the development process in parallel rather than in sequence as is usually done. You can think of COVID-19 vaccine development steps being completed like several cars traveling in separate lanes of a multilane highway. In contrast, during the more traditional vaccine development process steps are completed like cars of a train, one after the other. This change of process, coupled with the financial resources provided through Operation Warp Speed, enabled this extraordinary scientific achievement. The speed with vaccines were made saved millions of lives.

The disease

Even five years after SARS-CoV-2 emerged, the virus continues to cause millions of illnesses, hundreds of thousands of hospitalizations, and tens of thousands of deaths each year in the U.S. COVID-19 can also lead to lingering symptoms, commonly called “long COVID.” This condition is not yet completely understood.

What is SARS-CoV-2?

SARS-CoV-2 is the virus that causes COVID-19. SARS-CoV-2 was first identified in China in late 2019. One source of confusion about SARS-CoV-2 is how it started. While we can’t know for certain, an overwhelming amount of evidence suggests this virus evolved from an animal infection. Some people say it was leaked from a lab in China, but most scientists agree that the evidence of viral evolution from an animal makes that scenario more likely.

The evidence suggests that the new virus developed when a bat coronavirus infected another animal, which has yet to be identified, and changed in a way that enabled it to gain two important capabilities. First, it could infect people and cause them to become ill. Second, it could easily spread from one person to another. These new capabilities set the stage for a global epidemic, called a pandemic. 

Watch as Dr. Offit discusses the origins of SARS-CoV-2 virus and why the so-called “lab leak theory” is unlikely.

While it may seem more satisfying to think that the actions of people caused the COVID-19 pandemic because it feels more controllable, the satisfaction is likely to be short-lived. We must be aware that new viruses can emerge at any time and cause global emergencies, such as we witnessed with COVID-19. This is increasingly the case as we live, work and play in closer proximity to animals. For more information about the interactions of people with viruses and other potential pathogens, check the article, “Opportunity Knocks: When Humankind Opens the Door to Disease.”

In March 2020, the World Health Organization determined that the virus had spread throughout the world and declared a COVID-19 pandemic. Since then, the virus has continued to change. Different versions are called “variants.” Variants have genetic differences compared with each other. The most successful variants of a virus tend to be those that either improve the ability of the virus to spread or allow it to evade the immune response generated by vaccination or previous illness.

What are the symptoms of COVID-19?

People with COVID-19 can have a wide range of symptoms. Some people never have symptoms. They may not even know they were infected. Most people have mild symptoms, similar to those of a cold. Others will develop severe symptoms that can result in weeks of illness, hospitalization or death. 

Common symptoms of COVID-19 include:

• Fever
• Dry cough
• Fatigue

Other symptoms can include:

• Loss of taste or smell
• Nasal congestion
• Conjunctivitis (“pink eye”)
• Sore throat
• Headache
• Muscle or joint pain
• Nausea or vomiting
• Diarrhea
• Loss of appetite
• Rash or discoloration of fingers or toes

Anyone can experience severe COVID-19. Some groups of people, are at greater risk, including:

• Those 65 years of age and older
• Pregnant women
• Individuals with high-risk medical conditions, such as high blood pressure, chronic heart and lung disorders, diabetes and obesity
• People with immune-compromising conditions

Symptoms of severe disease can include:

• Shortness of breath or difficulty breathing
• Confusion
• Chest pain
• High fever
• Blood clots

Complications

Some complications are also worth noting.

Myocarditis: Myocarditis is an inflammation of the heart muscle. People with COVID-19 can experience myocarditis. Early during the pandemic, rates of myocarditis were 16 times greater in people sick with COVID-19 compared with people who did not have COVID-19. It is unlikely that the rates continue to be as high with more recent variants of the virus, but data are needed to confirm the current rates following infection. 

MIS-C or MIS-A: Some children experience a delayed onset of symptoms called multisystem inflammatory syndrome (MIS-C). Adults might suffer a similar problem called MIS-A. This syndrome can occur in anyone who was infected with COVID-19, including people who had minimal or no symptoms during infection. The condition develops a few weeks after infection (up to 6 weeks) when individuals no longer test positive for SARS-CoV-2. Symptoms may include fever, abdominal pain, vomiting, diarrhea, rash, pink eye, fatigue, neck pain, or headache. It is not known why this syndrome develops. But because the virus can infect various types of cells, people with MIS-C or MIS-A often experience symptoms involving several organs or parts of the body, like the heart, blood, lungs, kidneys, eyes, stomach, intestines, or skin. This condition has become less common as new variants of the virus have emerged. When SARS-CoV-2 emerged, the rate of MIS-C was about 6 or 7 cases per 1 million people. But in 2024, the rate was about 8 cases per 100 million people. 

Long COVID: Some people experience side effects for long periods after infection (months or years). Symptoms of “long COVID” can include fatigue, difficulty thinking or concentrating (so-called “brain fog”), headache, change in or loss of taste or smell, dizziness, heart palpitations, chest pain, shortness of breath, cough, joint or muscle pain, anxiety, depression, sleep problems, feelings like “pins and needles,” diarrhea or stomach pain, rash, changes in menstrual cycle or fever. Symptoms may appear or worsen after physical or mental activity. Some people, particularly those who had severe disease, may develop new chronic conditions, like diabetes, heart conditions or neurological conditions. We are still learning why this happens and how long the effects may linger. Currently, it seems that those more likely to experience lingering symptoms include people who:

• Had more severe disease (e.g. were hospitalized or put in intensive care)
• Developed MIS-C or MIS-A
• Had pre-existing health conditions
• Were unvaccinated

How do you catch SARS-CoV-2?

Coronaviruses are contained in saliva. They spread by talking, coughing, sneezing or singing. These respiratory secretions are usually “large droplets” that do not stay in the air for long periods of time. However, smaller virus-containing droplets, sometimes referred to as “aerosolized droplets,” can stay in the air for long periods of time. This makes them more contagious. Virus-containing respiratory droplets can enter a person’s eyes, nose or mouth. Once in the body, the virus can infect cells that line the nose, throat, lungs and intestines.

How can you avoid catching SARS-CoV-2?

Public health measures, like handwashing, social distancing or masking can help lower the chance of getting COVID-19 or other respiratory viruses. But humankind has never eliminated an infection solely with public health measures. Therefore, COVID-19 vaccines remain an important tool. They can help people have specific immunity against the virus that causes COVID-19, and they can help decrease the spread of the virus in communities. Decreasing spread of the virus can help limit the development of new variants. 

Some people feel the COVID-19 vaccines do not work because people can get infected even after vaccination. A couple of points are worth understanding related to this: 

• We often get infected with respiratory viruses more than once throughout our lifetimes. This is because of how our immune systems work. When we have immunologic memory, it takes a few days to be activated and stop an infection. During that period, the virus continues to replicate. People may experience minor respiratory symptoms.
• Given the way that our immune systems work, vaccination is not likely to completely prevent infections either. Vaccination guides our immune systems to develop immunologic memory in a controlled way, so we don’t get severely ill or die the first time our body sees the germ or pathogen. Immunologic memory from vaccination will work the same way as immunologic memory from an infection, meaning there will be a delay of a few days during activation of the immune memory. During this time, the virus will replicate.
• So, why vaccinate? In addition to controlling the severity of the first infection, vaccination helps make people less likely to become severely ill compared with non-immune people during future encounters with the pathogen.
• For COVID-19, we have learned that those who have been both vaccinated and infected appear to have the best immunologic protection, so even people who were infected can benefit from vaccination. 

The vaccine

How is the COVID-19 vaccine made?

The COVID-19 vaccines currently available in the U.S. include three mRNA vaccines and one recombinant, protein-based vaccine.

• mRNA vaccines — These types of vaccines provide the code, or blueprint, for making viral proteins. COVID-19 mRNA vaccines provide the blueprint for the SARS-CoV-2 spike protein. Specialized immune system cells near the injection site make the SARS-CoV-2 spike protein. The immune system recognizes the protein as foreign and responds to remove it. In the process, the body creates a memory immune response. If the person is exposed to the virus that causes COVID-19 in the future, their immune system will respond more quickly. Watch this animation to see how the COVID-19 mRNA vaccine works. 

  Three COVID-19 mRNA vaccines are currently available. All are given as shots into muscle (called intramuscular injections).

  • Comirnaty (Pfizer) – Given in one to three doses. This vaccine is approved for use in people 12 years and older.
  • Spikevax (Moderna) – Given in one to three doses. This vaccine is approved for use in people 65 years and older and those between 6 months and 64 years of age at increased risk for severe COVID-19.
  • Mnexspike (Moderna) – Instead of containing the code for the whole spike protein, this version contains the code for two important parts of the spike protein, called the N-terminal domain (NTD) and the receptor-binding domain (RBD). This vaccine is approved for use in people 65 years and older and those between 12 and 64 years at increased risk for severe COVID-19.
  • Many individuals are recommended to get additional doses of mRNA vaccine. Talk to your healthcare provider if you are uncertain about how many doses you are recommended to receive. You can also find more information on this page of our website in the answer to “Who should get a booster dose of COVID-19 vaccine?”
• Recombinant, protein-based vaccine – The SARS-CoV-2 spike protein is made in an insect-cell system in the lab. It is then purified for use as the vaccine. An adjuvant is added to enhance the immune response to the single protein. It is the same adjuvant used in the shingles vaccine. This method of making the vaccine is similar to that used for other routinely recommended vaccines, like shingles, HPV, hepatitis B, and one type of influenza vaccine (Flublok®).
  • Nuvaxovid (Novavax) – One recombinant COVID-19 vaccine is available in the U.S. This vaccine is given as one dose administered as a shot in the muscle. This vaccine is approved for use in people 65 years and older and those between 12 and 64 years at increased risk for severe COVID-19.
• Adenovirus vaccines are no longer available in the U.S due to some serious side effects that they cause and the availability of alternative options. However, this type of COVID-19 vaccine is available in some other countries. These types of vaccines use an adenovirus as a delivery vehicle for the gene that codes for the SARS-CoV-2 spike protein. The adenovirus has not only been altered to include the spike protein gene, but it has also been changed so that it cannot replicate in people. Once in the cell, the adenovirus DNA, including the DNA for the coronavirus spike protein, enters the nucleus, where it is used to make mRNA. The mRNA is then released to the cytoplasm where it serves as a blueprint to produce proteins, including the SARS-CoV-2 spike protein. The immune system recognizes the protein as foreign and responds to remove it. In the process, the body creates a memory immune response. If the person is exposed to the virus that causes COVID-19 in the future, their immune system will respond more quickly. Watch this animation to see how the COVID-19 viral vector vaccine works.

For information about COVID-19 vaccine ingredients, check out the “COVID-19 Vaccines: What’s in the Vial?” infographic or accompanying article.

Does the COVID-19 vaccine have side effects?

Some people may feel tired after getting the COVID-19 vaccine, and some will have a headache, muscle aches, or injection site pain. Some will also develop a fever, and some may experience swelling of the lymph nodes under the arm in which the vaccine was given.

The side effects are more common after the second dose. They are also more likely in younger, rather than older, vaccine recipients. The side effects can occur up to a week after vaccination but most often occur one or two days after vaccination. The side effects usually last for a day or two. Side effects in young children were similar to those in adults. These types of side effects are the result of the immune system responding to the vaccine.

A small number of people, particularly young men, can experience a temporary inflammation of the heart, called myocarditis. If this happens, it usually occurs within 4 days of receipt of the second dose of the mRNA vaccine. Affected people can feel chest pain or shortness of breath. They should seek medical care to ensure proper diagnosis. The condition most commonly goes away in a few days and does not appear to cause permanent damage to the heart, but it can be scary. Infection with SARS-CoV-2 can more frequently cause myocarditis, either during primary infection or later if individuals develop multisystem inflammatory syndrome (MIS-C or MIS-A). Therefore, vaccination is usually considered to be the safer choice. 

When the vaccines were first available, myocarditis was estimated to occur in about 1 of 50,000 people. Most often, this occurred in males in their teens and 20s after receiving the second dose of the mRNA vaccine. Over time, the rates of myocarditis after vaccination have decreased. In fact, the rates have not been above background for the last three COVID-19 vaccine seasons (2022-2023, 2023-2024, and 2024-2025). Also, myocarditis has not been detected in younger children, who receive lower doses of the vaccine. However, parents should continue to monitor their children after receipt of the vaccine until we know that this rare side effect does not also occur in younger children. Myocarditis is also likely a rare side effect of the Novavax vaccine.

Adenovirus-based viral vector vaccines are no longer available in the U.S. This is because they can cause two concerning side effects:

• Thrombosis with thrombocytopenia syndrome (TTS) — a very rare blood-clotting condition that also causes low platelet counts. The condition occurs in about 1 per 500,000 people who get the J&J/Janssen vaccine.
• Guillain-Barre syndrome (GBS) — this neurologic condition is characterized by muscle weakness. The condition can occur following a viral infection but has also been associated with receipt of the J&J/Janssen vaccine in about 1 per 120,000 people. 

Who should get the COVID-19 vaccine?

Almost everyone can get the COVID-19 vaccine, but vaccine decisions should be informed by the type of COVID-19 vaccination a person would be getting. Two types of COVID-19 vaccination have different considerations.

Previously unvaccinated people getting a “primary series” of vaccine

A “primary series” of vaccination refers to the doses of vaccine that create protective immunity. For some vaccines, like measles vaccine, the primary series is one dose (usually at 12-15 months of age). For other vaccines, like DTaP, the primary series is three doses (at 2 months, 4 months, and 6 months of age). The primary series for COVID-19 vaccines is between one and three doses, depending on the age of the person and the type of COVID-19 vaccine they get.

Because COVID-19 only emerged in 2019, we have two groups of previously unvaccinated people:

• People who lived through the pandemic but were never vaccinated – Most of those people will have some immunity to the virus that causes COVID-19 because they were likely infected at some point. These people could still benefit from getting a COVID-19 vaccine because we have found that people who have been both vaccinated and infected have the best immunity. However, even if this group opts not to get vaccinated, most of them will not get severely ill because they have some immunity already.
• Unvaccinated babies and children born since 2020 – Although COVID-19 tends to be less severe in children, severe cases and deaths still occur. This is because the virus that causes COVID-19 is similar to pathogens that cause other vaccine-preventable diseases. What does that mean? Three to four million new babies are born each year in the U.S. This new group does not have immunity to the virus, so they are more susceptible to infection. Since many of them will be getting infected, some will suffer severe disease that requires hospitalization. During the 2024-2025 COVID-19 season, children less than 5 years of age accounted for the most hospitalizations among those younger than 18 years with the youngest of those (less than 6 months) having the highest rates. Children between 6 and 23 months of age were the second most hospitalized group. Further, about half of the children less than 4 years of age who were hospitalized with COVID-19 during the 2024-2025 season were previously healthy. Therefore, unvaccinated children can benefit from a primary series.

Previously vaccinated people getting “booster doses” of vaccine

The second type of COVID-19 vaccination is “booster doses,” meaning additional doses after receiving the primary series. Because the virus that causes COVID-19 has continued to change, the vaccines have changed as well. One result of these changes has been recommendations for annual booster doses. Beginning in 2025-2026, not everyone will be recommended to get these boosters. 

The following groups are most likely to benefit from booster doses:

• Those 65 years of age and older
• Pregnant women
• Individuals with high-risk medical conditions, such as high blood pressure, chronic heart and lung disorders, diabetes and obesity
• People with immune-compromising conditions

These individuals are at increased risk for severe COVID-19, so the boost in immunity can help decrease that possibility. Some of these individuals may actually benefit from more than one booster dose. Check with your healthcare provider if you are not sure if you should get more than one booster dose.

Others who have had the primary series may also benefit from a booster dose, particularly people caring for high-risk individuals and those who want to boost their own protective immunity.

Who should not get or has special considerations related to COVID-19 vaccine?

A few people should not get it, and some others should first consult with their healthcare provider.

People who should NOT get the COVID-19 vaccine include:

• Those under 6 months of age. Those 6 months to 18 years of age can get the mRNA vaccines. The recombinant vaccine (Novavax) can only be used for those 12 years of age and older. Dosing for children differs based on age, brand of vaccine and, in some cases, pre-existing health conditions. Talk with your child’s healthcare provider if you are unsure about the recommendations for your child.
• People with COVID-19 should wait until they recover. These individuals may opt to delay vaccination for about three months after infection to improve their immune response to the vaccine.

People who cannot get the mRNA vaccine, but may be able to get the recombinant vaccine:

• Anyone with a previous severe or immediate allergic reaction to a COVID-19 mRNA vaccine dose or an mRNA vaccine component. This includes a reaction that causes anaphylaxis, including swelling of the airway (i.e., tongue, uvula, or larynx). It also includes any reaction with a diffuse rash that also involves respiratory surfaces (e.g., Stevens-Johnson Syndrome).
• Anyone with a known allergy to polyethylene glycol.

People who cannot get the recombinant (protein-based) vaccine, but may be able to get the mRNA vaccine:

• Anyone with a previous severe or immediate allergic reaction to the COVID-19 recombinant vaccine or one of its components. This includes a reaction that causes anaphylaxis, including swelling of the airway (i.e., tongue, uvula, or larynx). It also includes any reaction with a diffuse rash that also involves respiratory surfaces (e.g., Stevens-Johnson Syndrome).
• Anyone with a known polysorbate allergy.
• Those younger than 12 years of age.

People who may get the vaccine but should be aware of the potential risks and benefits (through discussions with a healthcare provider, as necessary) or who should follow special procedures:

• Individuals with a history of a non-severe, immediate (within 4 hours) allergic reaction to a previous dose of COVID-19 vaccine should stay at the location where the vaccine is given for 30 minutes after receiving it. Speak with your doctor before vaccination if you have questions or concerns.
• People who have a severe or immediate allergic reaction to one of the types of vaccines and for whom the cause of the reaction is unknown (i.e., which component caused the reaction) should consult an allergist or immunologist to determine whether they can get the other version. If they proceed with vaccination, they should be vaccinated at a location with medical facilities and staff prepared to respond to medical emergencies. They should stay at the location for 30 minutes after getting the vaccine.
• Pregnant people who develop a fever after vaccination should take acetaminophen as fever during pregnancy can be harmful to the developing baby.
• People who are moderately or severely ill (regardless of whether they have a fever) may delay vaccination until they feel better.
• People with a history of MIS-C or MIS-A should delay vaccination until at least 90 days after diagnosis. They should also have a healthcare provider confirm a return of normal cardiac function and be considered clinically recovered.
• People who developed myocarditis or pericarditis within three weeks after receipt of mRNA vaccine are typically advised not to get additional doses of any COVID-19 vaccine. In some instances, individuals and their healthcare providers may decide to proceed with an additional dose based on the risk-benefit assessment. Note: This does not apply to people with history of myocarditis or pericarditis unrelated to COVID-19 vaccination (including from COVID-19 infection prior to vaccination or more than three weeks after COVID-19 vaccination). Nor does it apply to people with a history of heart disease.
• People with COVID-19 who are treated with convalescent plasma should not receive measles- or varicella-containing vaccines until at least seven months after receipt of the plasma.
• People with a known COVID-19 exposure can get vaccinated if they do not have symptoms.
• People with a current infection can get vaccinated after they recover, but they may prefer to wait until at least three months after the start of symptoms to develop a better immune response to the vaccine.

All individuals should stay at the vaccination site for 15-30 minutes of observation after getting vaccinated.

Other questions you might have

Find answers to dozens of questions related to COVID-19 vaccines on our dedicated page, COVIDVaccineAnswers.org.

If I already had COVID-19, should I still get vaccinated?

Yes. Studies have found that people who were both vaccinated and infected are among the best protected because the vaccine adds to the breadth of protection the person has.

If I get the COVID-19 vaccine, do I still need to get the flu shot?

Yes. While COVID-19 and influenza are both caused by respiratory viruses, the viruses are different. So, separate vaccines are needed to protect you from both viruses.

Will the cost of the COVID-19 vaccine be covered by my insurance since the recommendations changed in 2025?

It is unclear whether all insurance companies will cover the cost of COVID-19 vaccines for all people in 2025. AHIP is the professional organization for the insurance industry. In June 2025, they indicated that they would continue covering vaccines during the upcoming respiratory virus season. Hopefully, this means most of the companies they represent will cover COVID-19 vaccines, but there could be variability in coverage. If you are uncertain, talk to your insurance provider before going for the vaccine. Check about two different potential charges: the cost of the vaccine and any administration fee. Also check if there is a particular location you need to get the vaccine for it to be covered (e.g., at your healthcare provider office or at a pharmacy). 

If you don’t have coverage or if your insurance is not covering the costs associated with vaccination, check with your local health department. They may be aware of free or reduced cost vaccine. You can also check with different pharmacies in your area to compare prices.

Should my healthy infant or child get the COVID-19 vaccine? 

COVID-19 is usually mild for children, but that is not always the case. As such, a few considerations are worthwhile when thinking about the COVID-19 vaccine for children:

• Infants turning 6 months of age – Most children hospitalized with COVID-19 are the youngest children. This is because they have not been exposed to the virus that causes COVID-19. Any protection from maternal antibodies is likely waning, and they may not yet have developed their own immunity. For these reasons, this group can benefit from getting the COVID-19 vaccine.
• Young children (under 5 years of age) not previously vaccinated – Even if a child has had COVID-19, they can benefit from COVID-19 vaccination. This is because the vaccination can strengthen and diversify their immunity against the virus.
• Older children previously vaccinated – Healthy children who previously received a complete primary series of COVID-19 vaccine can get a vaccine booster if you want to give them additional protection. But, generally, COVID-19 is mild if they are infected and if they have had the vaccine previously, they will have immunologic memory that is likely to protect them against developing severe illness. If your older child is in a risk group because of other health conditions, they would benefit from getting another dose of vaccine. If they are not in a risk group, the choice could be considered “low risk, low reward.”
• Older children not previously vaccinated – Older children who never got a COVID-19 vaccine series likely have some immunity due to exposure to the virus in the community. Since the vaccine makes for stronger immunity, they could benefit from getting a dose, but the gain is less than for some of the other groups of children.

Notably, myocarditis associated with vaccination has decreased over time. During the last three COVID-19 vaccination seasons (2022-2023, 2023-2024, and 2024-2025), rates of myocarditis after receipt of the COVID-19 vaccine have been similar to background rates.

Also, if children have any of the conditions that increase the risk for adults, they are also considered high-risk and would be recommended to get booster doses.

Relative risks and benefits

Do the benefits of COVID-19 vaccination outweigh its risks?

COVID-19 can cause severe disease or even death. Some people may develop a multisystem inflammatory syndrome (MIS-C or MIS-A) several weeks after infection or long-lasting effects (“long COVID”). Long COVID results in symptoms that last for weeks or months, even when the virus can no longer be detected in the body.

On the other hand, the COVID-19 vaccine has not been shown to cause serious side effects in most people. Even though a small number of people, particularly young males, might experience short-lived inflammation of the heart (myocarditis) following receipt of the mRNA vaccine (and likely after the recombinant vaccine), this condition resolves within a few days and is less severe than the heart inflammation associated with COVID-19 infection. 

Disease risks

• Pneumonia (infection of the lungs)
• Respiratory failure
• Acute respiratory distress syndrome (ARDS)
• Sepsis (bloodstream infection) and septic shock
• Multi-organ failure
• Children and adults can develop a rare, yet severe, inflammatory disorder weeks after infection, called multisystem inflammatory syndrome (MIS-C or MIS-A)
• Long-term symptoms (“long COVID”)
• Disease can be fatal

Vaccine risks

• Fatigue
• Headache
• Muscle aches
• Swelling of the lymph nodes under the arm in which the vaccine was given
• Myocarditis (previously 1 per 50,000 doses, but has not been found to be elevated during the three recent vaccine seasons)
• Anaphylaxis (3-5 per 1 million people)

References

• Coronavirus disease (COVID-19) – World Health Organization
• Orenstein WA, Offit PA, Edwards KM and Plotkin SA. Diphtheria Toxoid in Plotkin's Vaccines, 8th Edition. 2024, 248-257.

Reviewed by Paul A. Offit, MD on August 11, 2025