Should my child get an early second dose of measles vaccine during the current outbreaks?
Why do we need a flu vaccine every year? That’s like 18 doses during childhood alone.
Why does the federal government now only recommend one dose of HPV vaccine?
We get a fair number of questions about vaccine doses. Sometimes people are asking about the safety of extra doses, and sometimes they are asking about what to do if doses weren’t completed according to the schedule. Over time, we have addressed many of these types of dose-related questions. However, this month, we thought it might be helpful to talk about the science behind vaccine dosing to get at some of the “why” questions that people may wonder about when it comes to vaccine doses.
Vaccine dosing is not arbitrary. It is part of the clinical trial process. Typically, the number of doses and the quantity of antigen are part of early clinical trials (phase I and phase II). By the time large phase III trials are conducted, the number and strength of doses have been determined. Several factors are considered when it comes to deciding on the number of vaccine doses. Some of these factors relate to the vaccine, such as the type of vaccine platform (e.g., live, weakened or inactivated) and the strength of immune responses generated by vaccination. Some considerations relate to the pathogen, such as its characteristics and disease epidemiology. Further complicating this science is that dosing regimens may change over time. So, let’s use some vaccine stories to see how all of this fits into the way a particular vaccine is recommended.
Hepatitis B: The role of the second and third doses
Most vaccines are given as multiple doses. This is particularly true of vaccines that are made from whole inactivated viruses or single viral proteins, meaning they contain proteins or sugars that do not replicate after being administered. Even if these doses are administered in higher quantities, the reality is that they often do not activate all parts of the immune system in the same way that a pathogen or live, weakened vaccine does. Likewise, adjuvants can help boost the immune response so that fewer doses are needed, but often, even then, multiple doses are necessary.
Let’s look at the hepatitis B vaccine. It is a recombinant vaccine made of a single protein from hepatitis B virus. The vaccine is typically recommended as a three-dose series with the second dose given one to two months after the first dose, and the third dose at least six months after the first dose. Often when doses are scheduled closer in time, like doses one and two of the hepatitis B vaccine series, they are considered primary doses, meaning they are contributing to the initial development of immunity against that pathogen. In scientific terms, these kinds of doses are contributing to “primary immunity.” When doses are further delayed in time, such as the third dose of hepatitis B vaccine, they are more often reminding the immune system about the pathogen. Scientifically speaking, they are contributing to “memory immunity.” Now, what’s important to realize here is that our immune systems generate memory immunity after any immune response, so even after those first two doses, most people will have some memory immunity, but every time our immune system comes into contact with the same pathogen, it gets better at recognizing it. Think of it like studying. When students are studying, they get better at remembering and understanding the information each time they encounter it. The same is true of our immune systems. By waiting for a few months, the cells involved in the primary immune response will go away, but the memory cells will remain. So, when the third dose of vaccine is given, it is the memory cells that drive the immune response. After which, the cells that comprise memory immunity will be even better prepared to protect against hepatitis B than those that were present before the third dose.
Since the vaccine was studied with all three doses and because that third dose strengthens memory immunity, giving fewer doses — even if memory immunity is measurable — means that a person who had one or two doses may not enjoy protection that is as robust or as long-lasting as that of someone who has had three doses.
Measles: The role of the second dose
While all subsequent doses of a vaccine are likely to boost a person’s memory immunity, that’s not always the main goal of additional doses. Measles vaccine offers an example. Measles vaccine is given in two doses: the first around 1 year of age, and the second between 4 and 6 years of age before starting school. It is a live, weakened viral vaccine, so when a person is vaccinated, the virus replicates and the immune system responds to it. After one dose of measles vaccine, about 93 of every 100 people will develop robust memory immunity that will protect them throughout life. The other seven will not. In most cases, these people will have some immunity, but they won’t necessarily be protected during an outbreak. We can’t know which seven people out of the 100 will find themselves in this situation.
By giving a second dose right before children start school, we accomplish two things. We remind the immune system about measles, so memory immunity is boosted for each vaccinated individual, but the main reason for the second dose is to give those seven people who did not develop lifelong protection another chance to do so. After the second dose, 97 of the 100 people will have protection that is lifelong; three still will not. These three will rely on community immunity to reduce the chance for exposure to the virus. We won’t know which three of the original 100 people are in this situation unless an outbreak occurs and they become ill.
Influenza: The need for annual doses
Sometimes, the nature of the pathogen is central to dosing decisions. Influenza offers an example. Most influenza vaccines are inactivated or recombinant, single viral protein vaccines. The nasal spray is a live, weakened viral vaccine. None of the versions provide long-term protection. This is because protection against influenza is based on having our immune systems recognize proteins on the surface of influenza virus, and influenza virus is notorious for changing those proteins as it reproduces. The virus changes so much, in fact, that sometimes in the months between when it travels from the Southern Hemisphere to the Northern Hemisphere, it has changed enough that our vaccines are not as efficient at inducing immunity. As a result of the ever-changing nature of the virus, vaccination one year does not mean someone will have protection the next year. It is for this reason that annual doses are recommended.
HPV: The ability to decrease doses
The human papillomavirus, or HPV, vaccine offers an interesting story when we are thinking about the number of doses of a vaccine. HPV vaccine protects against nine of the most common disease-causing types of HPV. For each type in the vaccine, a single protein from the surface of the virus is used. The protein used in the vaccine folds itself to look like the virus, but it can’t reproduce because it does not include genetic material. Importantly, because the protein folds to look like the virus, the immune system makes a robust response to it.
When the vaccine was first developed, it was tested and used as three doses in a timing regimen similar to the hepatitis B vaccine: 0, 1-2 months and 6 months. So, in scientific terms: two primary doses and a memory dose. However, because the vaccine is expensive and it is given to adolescents and teens, it was difficult to introduce in countries with limited resources and a lack of health infrastructure for routine adolescent healthcare visits. In an effort to overcome some of these barriers, scientists conducted studies in other countries to determine if they could give fewer doses. The earliest studies showed that two doses (one primary and one memory) worked in younger people, but older people still benefited more from three doses. This is reflected in the HPV recommendations that were practiced in the U.S. until recent federal changes to the schedule. Specifically, for those vaccinated between 9 and 14 years of age, they received two doses with the second one being given at least six months after the first dose. Those 15 and older still received three doses with the second dose at one to two months, and the third dose at least six months after the first dose.
More recent studies in other countries have suggested that one dose may be sufficient. Prior to the committee’s dismissal in June 2025, the Advisory Committee on Immunization Practices (ACIP), an advisory group to the Centers for Disease Control and Prevention (CDC), was prepared to review the global data to determine whether a further decrease in doses would make sense in the U.S. That is to say, for people in the U.S., is the primary dose without the memory dose sufficient to afford long-lived protection? When the CDC published its revised schedule in January 2026, HPV vaccine was changed to a single dose. Unfortunately, no evidence or discussion behind that change was made public, so it is unclear whether the change is supported by the data. The Vaccine Integrity Project (VIP) has convened a committee to review these data, and it may well be sufficient for us to use a single dose in the U.S. too, but we really need to see what these data support before we can rest assured that further decreasing the number of doses is the best recommendation for people in this country. For now, until the evidence is clear that a single dose of the HPV vaccine is adequate, it is probably best to stick with the original CDC recommendation of two doses for those between 9 and 14 years of age and three doses for those 15 and older.
RSV: The story of disappearing risk
Infants in the first year of life are particularly susceptible to respiratory syncytial virus, or RSV. Over the last few years, a couple of options for protecting them have become widely available. One approach is to vaccinate mom during pregnancy if the baby will be born during RSV season, so she shares maternal antibodies with the baby before birth. The second approach is to give the baby an antibody mixture right after birth if they are born during the season or right before their first RSV season if they are born outside of RSV season. In each of these scenarios, we are protecting babies by passive immunity, meaning they are protected by antibodies that their immune system did not make. Passive immunity is short-lived; typically, these antibodies last a few months. So, why are we protecting babies in such a fleeting way?
The answer lies in the fact that RSV is common, so babies are likely to be exposed during their first season. By ensuring that they have antibodies during that first exposure, they will be unlikely to get severely ill, but their immune system will still make its own immune response. So, by the time they reach their second RSV season, they have immunity that will most often protect them from severe disease. Notably, a small number of babies are recommended to get the antibody treatment again during their second RSV season, but this recommendation is limited to babies who have conditions that increase the chance that they won’t be able to protect themselves against severe disease.
Shingles: The story of increasing risk
Shingles is caused by the same virus that causes chickenpox. When a person gets chickenpox, the virus will remain in their nerves. The same occurs following vaccination because the vaccine is a live, weakened viral vaccine. In both cases, the virus lives silently in their body for decades, albeit in lesser quantities and a weaker version after vaccination compared with disease. However, as the person ages or if they have weakened immunity, the virus can awaken and start to replicate again resulting in a bout with shingles.
Chickenpox vaccine is given as two doses: one dose around 1 year of age and a second dose between 4 and 6 years of age around the time of starting school. The first dose of this vaccine prevents almost all cases of severe disease in all recipients, but about 1 or 2 of every 10 could experience mild disease if they are exposed to the virus. The second dose boosts memory immunity in all vaccine recipients and in doing so, it also decreases the likelihood for having mild disease if exposed, although a few people may still develop mild disease after two doses.
Since the majority of individuals are protected against chickenpox, they don’t get additional doses; however, as they age and the risk for shingles increases, they are recommended to get the shingles vaccine. The shingles vaccine is given to those 50 and older as two doses separated by two to six months. While an earlier version of shingles vaccine used the same virus as that in the chickenpox vaccine, the doses contained 14 times more virus to generate protective immunity against shingles due to the aging immune system of vaccine recipients. The currently available version, known as Shingrix, includes adjuvants that improve the immune response to a single protein from the surface of the virus. In this situation, the goal of both doses is to boost memory immunity, but the second dose is given, removed in time from the first dose, so that if the first dose generates immune responses that are lower, meaning more like primary responses, the second dose can still enhance memory immunity.
In sum
As demonstrated by the differences considered in the vaccine dosing strategies described, determining the number of doses necessary for a vaccine is nuanced and influenced by a variety of considerations. For these reasons, it is important to find out how many doses are recommended for a particular vaccine you or a family member is receiving and the suggested timing for any subsequent doses. This will ensure the best protection.
Should my child get an early second dose of measles vaccine during the current outbreaks?
Why do we need a flu vaccine every year? That’s like 18 doses during childhood alone.
Why does the federal government now only recommend one dose of HPV vaccine?
We get a fair number of questions about vaccine doses. Sometimes people are asking about the safety of extra doses, and sometimes they are asking about what to do if doses weren’t completed according to the schedule. Over time, we have addressed many of these types of dose-related questions. However, this month, we thought it might be helpful to talk about the science behind vaccine dosing to get at some of the “why” questions that people may wonder about when it comes to vaccine doses.
Vaccine dosing is not arbitrary. It is part of the clinical trial process. Typically, the number of doses and the quantity of antigen are part of early clinical trials (phase I and phase II). By the time large phase III trials are conducted, the number and strength of doses have been determined. Several factors are considered when it comes to deciding on the number of vaccine doses. Some of these factors relate to the vaccine, such as the type of vaccine platform (e.g., live, weakened or inactivated) and the strength of immune responses generated by vaccination. Some considerations relate to the pathogen, such as its characteristics and disease epidemiology. Further complicating this science is that dosing regimens may change over time. So, let’s use some vaccine stories to see how all of this fits into the way a particular vaccine is recommended.
Hepatitis B: The role of the second and third doses
Most vaccines are given as multiple doses. This is particularly true of vaccines that are made from whole inactivated viruses or single viral proteins, meaning they contain proteins or sugars that do not replicate after being administered. Even if these doses are administered in higher quantities, the reality is that they often do not activate all parts of the immune system in the same way that a pathogen or live, weakened vaccine does. Likewise, adjuvants can help boost the immune response so that fewer doses are needed, but often, even then, multiple doses are necessary.
Let’s look at the hepatitis B vaccine. It is a recombinant vaccine made of a single protein from hepatitis B virus. The vaccine is typically recommended as a three-dose series with the second dose given one to two months after the first dose, and the third dose at least six months after the first dose. Often when doses are scheduled closer in time, like doses one and two of the hepatitis B vaccine series, they are considered primary doses, meaning they are contributing to the initial development of immunity against that pathogen. In scientific terms, these kinds of doses are contributing to “primary immunity.” When doses are further delayed in time, such as the third dose of hepatitis B vaccine, they are more often reminding the immune system about the pathogen. Scientifically speaking, they are contributing to “memory immunity.” Now, what’s important to realize here is that our immune systems generate memory immunity after any immune response, so even after those first two doses, most people will have some memory immunity, but every time our immune system comes into contact with the same pathogen, it gets better at recognizing it. Think of it like studying. When students are studying, they get better at remembering and understanding the information each time they encounter it. The same is true of our immune systems. By waiting for a few months, the cells involved in the primary immune response will go away, but the memory cells will remain. So, when the third dose of vaccine is given, it is the memory cells that drive the immune response. After which, the cells that comprise memory immunity will be even better prepared to protect against hepatitis B than those that were present before the third dose.
Since the vaccine was studied with all three doses and because that third dose strengthens memory immunity, giving fewer doses — even if memory immunity is measurable — means that a person who had one or two doses may not enjoy protection that is as robust or as long-lasting as that of someone who has had three doses.
Measles: The role of the second dose
While all subsequent doses of a vaccine are likely to boost a person’s memory immunity, that’s not always the main goal of additional doses. Measles vaccine offers an example. Measles vaccine is given in two doses: the first around 1 year of age, and the second between 4 and 6 years of age before starting school. It is a live, weakened viral vaccine, so when a person is vaccinated, the virus replicates and the immune system responds to it. After one dose of measles vaccine, about 93 of every 100 people will develop robust memory immunity that will protect them throughout life. The other seven will not. In most cases, these people will have some immunity, but they won’t necessarily be protected during an outbreak. We can’t know which seven people out of the 100 will find themselves in this situation.
By giving a second dose right before children start school, we accomplish two things. We remind the immune system about measles, so memory immunity is boosted for each vaccinated individual, but the main reason for the second dose is to give those seven people who did not develop lifelong protection another chance to do so. After the second dose, 97 of the 100 people will have protection that is lifelong; three still will not. These three will rely on community immunity to reduce the chance for exposure to the virus. We won’t know which three of the original 100 people are in this situation unless an outbreak occurs and they become ill.
Influenza: The need for annual doses
Sometimes, the nature of the pathogen is central to dosing decisions. Influenza offers an example. Most influenza vaccines are inactivated or recombinant, single viral protein vaccines. The nasal spray is a live, weakened viral vaccine. None of the versions provide long-term protection. This is because protection against influenza is based on having our immune systems recognize proteins on the surface of influenza virus, and influenza virus is notorious for changing those proteins as it reproduces. The virus changes so much, in fact, that sometimes in the months between when it travels from the Southern Hemisphere to the Northern Hemisphere, it has changed enough that our vaccines are not as efficient at inducing immunity. As a result of the ever-changing nature of the virus, vaccination one year does not mean someone will have protection the next year. It is for this reason that annual doses are recommended.
HPV: The ability to decrease doses
The human papillomavirus, or HPV, vaccine offers an interesting story when we are thinking about the number of doses of a vaccine. HPV vaccine protects against nine of the most common disease-causing types of HPV. For each type in the vaccine, a single protein from the surface of the virus is used. The protein used in the vaccine folds itself to look like the virus, but it can’t reproduce because it does not include genetic material. Importantly, because the protein folds to look like the virus, the immune system makes a robust response to it.
When the vaccine was first developed, it was tested and used as three doses in a timing regimen similar to the hepatitis B vaccine: 0, 1-2 months and 6 months. So, in scientific terms: two primary doses and a memory dose. However, because the vaccine is expensive and it is given to adolescents and teens, it was difficult to introduce in countries with limited resources and a lack of health infrastructure for routine adolescent healthcare visits. In an effort to overcome some of these barriers, scientists conducted studies in other countries to determine if they could give fewer doses. The earliest studies showed that two doses (one primary and one memory) worked in younger people, but older people still benefited more from three doses. This is reflected in the HPV recommendations that were practiced in the U.S. until recent federal changes to the schedule. Specifically, for those vaccinated between 9 and 14 years of age, they received two doses with the second one being given at least six months after the first dose. Those 15 and older still received three doses with the second dose at one to two months, and the third dose at least six months after the first dose.
More recent studies in other countries have suggested that one dose may be sufficient. Prior to the committee’s dismissal in June 2025, the Advisory Committee on Immunization Practices (ACIP), an advisory group to the Centers for Disease Control and Prevention (CDC), was prepared to review the global data to determine whether a further decrease in doses would make sense in the U.S. That is to say, for people in the U.S., is the primary dose without the memory dose sufficient to afford long-lived protection? When the CDC published its revised schedule in January 2026, HPV vaccine was changed to a single dose. Unfortunately, no evidence or discussion behind that change was made public, so it is unclear whether the change is supported by the data. The Vaccine Integrity Project (VIP) has convened a committee to review these data, and it may well be sufficient for us to use a single dose in the U.S. too, but we really need to see what these data support before we can rest assured that further decreasing the number of doses is the best recommendation for people in this country. For now, until the evidence is clear that a single dose of the HPV vaccine is adequate, it is probably best to stick with the original CDC recommendation of two doses for those between 9 and 14 years of age and three doses for those 15 and older.
RSV: The story of disappearing risk
Infants in the first year of life are particularly susceptible to respiratory syncytial virus, or RSV. Over the last few years, a couple of options for protecting them have become widely available. One approach is to vaccinate mom during pregnancy if the baby will be born during RSV season, so she shares maternal antibodies with the baby before birth. The second approach is to give the baby an antibody mixture right after birth if they are born during the season or right before their first RSV season if they are born outside of RSV season. In each of these scenarios, we are protecting babies by passive immunity, meaning they are protected by antibodies that their immune system did not make. Passive immunity is short-lived; typically, these antibodies last a few months. So, why are we protecting babies in such a fleeting way?
The answer lies in the fact that RSV is common, so babies are likely to be exposed during their first season. By ensuring that they have antibodies during that first exposure, they will be unlikely to get severely ill, but their immune system will still make its own immune response. So, by the time they reach their second RSV season, they have immunity that will most often protect them from severe disease. Notably, a small number of babies are recommended to get the antibody treatment again during their second RSV season, but this recommendation is limited to babies who have conditions that increase the chance that they won’t be able to protect themselves against severe disease.
Shingles: The story of increasing risk
Shingles is caused by the same virus that causes chickenpox. When a person gets chickenpox, the virus will remain in their nerves. The same occurs following vaccination because the vaccine is a live, weakened viral vaccine. In both cases, the virus lives silently in their body for decades, albeit in lesser quantities and a weaker version after vaccination compared with disease. However, as the person ages or if they have weakened immunity, the virus can awaken and start to replicate again resulting in a bout with shingles.
Chickenpox vaccine is given as two doses: one dose around 1 year of age and a second dose between 4 and 6 years of age around the time of starting school. The first dose of this vaccine prevents almost all cases of severe disease in all recipients, but about 1 or 2 of every 10 could experience mild disease if they are exposed to the virus. The second dose boosts memory immunity in all vaccine recipients and in doing so, it also decreases the likelihood for having mild disease if exposed, although a few people may still develop mild disease after two doses.
Since the majority of individuals are protected against chickenpox, they don’t get additional doses; however, as they age and the risk for shingles increases, they are recommended to get the shingles vaccine. The shingles vaccine is given to those 50 and older as two doses separated by two to six months. While an earlier version of shingles vaccine used the same virus as that in the chickenpox vaccine, the doses contained 14 times more virus to generate protective immunity against shingles due to the aging immune system of vaccine recipients. The currently available version, known as Shingrix, includes adjuvants that improve the immune response to a single protein from the surface of the virus. In this situation, the goal of both doses is to boost memory immunity, but the second dose is given, removed in time from the first dose, so that if the first dose generates immune responses that are lower, meaning more like primary responses, the second dose can still enhance memory immunity.
In sum
As demonstrated by the differences considered in the vaccine dosing strategies described, determining the number of doses necessary for a vaccine is nuanced and influenced by a variety of considerations. For these reasons, it is important to find out how many doses are recommended for a particular vaccine you or a family member is receiving and the suggested timing for any subsequent doses. This will ensure the best protection.