Vaccine Education Center

Vaccines and the Immune System

Do vaccines overwhelm the immune system?

Studies on the diversity of antibody specificities indicate that the immune system has the capacity to respond to extremely large numbers of immunologically distinct regions of viruses and bacteria. Current data suggest that the theoretical capacity determined by the genes that make different antibodies would allow for as many as 109-1011 different kinds of antibodies (i.e., 1 billion to 100 billion). But this theoretical capacity is limited by the number of circulating antibody-producing cells (B cells or lymphocytes) and the likely redundancy of antibodies generated by one individual.

A more practical way to determine the diversity of the immune response would be to estimate the number of vaccines to which a child could respond at one time. Assuming the quantities of antibodies likely generated by an individual in 1 ml of blood (one-fifth of a teaspoon) during seven days after exposure to a vaccine, and the number of different specificities of those antibodies, then each infant would have the capacity to respond to about 10,000 vaccines at any one time. Using this estimate, one would predict that if 11 vaccines were given to infants at one time, then about 0.1 percent of the immune system would be "used up."

However, because B cells and other lymphocytes are constantly replenished, a vaccine never really "uses up" a fraction of the immune system. For example, the immune system has the ability to replenish about 2 billion lymphocytes each day. This replacement activity illustrates the enormous capacity of the immune system to generate lymphocytes as needed.

Parents may also take comfort in knowing that children are exposed to fewer immunologic components (like proteins and sugars [polysaccharides]) in vaccines today than in the past. The table below summarizes the number of proteins and polysaccharides contained in routinely recommended vaccines administered over the past 100 years. Although we now give children more vaccines, the actual number of immunologic components in vaccines has declined. Whereas previously one vaccine, smallpox, contained about 200 proteins, now the 14* routinely recommended vaccines contain about 160 immunologic components (i.e., proteins or polysaccharides). Two factors account for this decline: first, the worldwide eradication of smallpox obviated the need for that vaccine, and second, advances in protein chemistry have resulted in vaccines containing fewer antigens (e.g. replacement of whole-cell with acellular pertussis vaccine).

*Infants and young children receive vaccines to prevent 14 different diseases; some are given in combination.

Number of immunogenic proteins or sugars (polysaccharides) contained in vaccines over the past 100 years
Date Vaccine Proteins/Sugars Totals
1900 Smallpox ~200 ~200
1960 Smallpox ~200  ~3,217
  Diphtheria 1  
  Tetanus 1  
  Whole cell Pertussis ~3,000  
  Polio 15  
1980 Diphtheria 1  ~3,041
  Tetanus 1  
  Whole cell Pertussis ~3,000  
  Polio 15  
  Measles 10  
  Mumps 9  
  Rubella 5  
2000 Diphtheria 1 134-137
  Tetanus 1  
  Acellular pertussis 2-5  
  Polio 15  
  Measles 10  
  Mumps 9  
  Rubella 5  
  Hib 2  
  Varicella 69  
  Pneumococcus 8  
  Hepatitis B 1  
  Influenza 11  
Adapted from: Offit PA, et al. Addressing parents' concerns: Do vaccines weaken or overwhelm the infant's immune system? Pediatrics 2002;109:124-129.


Offit PA, Quarles J, Gerber MA, et al. Addressing parents’ concerns: Do multiple vaccines overwhelm or weaken the infant’s immune system? Pediatrics. 2002;109:124-9.

Tonegawa S, Steinberg C, Dube S, Bernardini A. Evidence for somatic generation of antibody diversity. Proc Natl Acad Sci USA. 1974;71:4027-31.

Cohn M, Langman RE. The protecton: the unit of humoral immunity selected by evolution. Immunol Rev. 1990;115:9-147.

Ramsay DS, Lewis M. Developmental changes in infant cortisol and behavioral response to inoculation. Child Dev. 1994;65:1491-502.


Do vaccines weaken the immune system?

Vaccinated children are not at greater risk of other infections (infections not prevented by the vaccines) than unvaccinated children. On the contrary, in Germany, a study of 496 vaccinated and unvaccinated children found that children who received immunizations against diphtheria, pertussis, tetanus, Haemophilus influenzae type b (Hib) and polio within the first three months of life had fewer infections with vaccine-related and unrelated pathogens than the non-vaccinated group.

Bacterial and viral infections, on the other hand, often predispose children and adults to severe, invasive infections with other pathogens. For example, children with pneumococcal pneumonia are more likely to have had a recent influenza infection than other children. Similarly, varicella infection increases susceptibility to the 'flesh-eating bacteria (i.e., group A strep).


Black SB, Cherry JD, Shinefield HR, et al. Apparent decreased risk of invasive bacterial disease after heterologous childhood immunization. Am J Dis Child 1991;145:746-749.

Davidson M, Letson W, Ward JI, et al. DTP immunization and susceptibility to infectious diseases. Is there a relationship? Am J Dis Child 1991;145:750-754.

Otto S, Mahner B, Kadow I, et al. General non-specific morbidity is reduced after vaccination within the third month of life — the Greifswald study. J Infect 2000;41:172-175.

Storsaeter J, Olin P, Renemar B, et al. Mortality and morbidity from invasive bacterial infections during a clinical trial of acellular pertussis vaccines in Sweden. Pediatr Infect Dis J 1988;7:637-645.


Can children who are sick receive vaccines?

Some parents may be concerned that children with acute illnesses are less likely to respond to vaccines or are more likely to develop severe reactions to vaccines than are healthy children. Alternatively, some parents may believe that children who are ill shouldn't further burden an immune system already committed to fighting an infection. However, vaccine-specific antibody responses and rates of vaccine-associated adverse reactions of children with mild or moderate illnesses are comparable to those of healthy children. For example, the presence of upper respiratory tract infections, ear infections, fever, skin infections or diarrhea does not affect the level of protective antibodies induced by immunization.

Data on the capacity of vaccines to induce protective immune responses in children with severe infections (such as those with bacterial pneumonia or meningitis) are lacking. Although a delay in vaccines is recommended for children with severe illnesses until the symptoms of illness resolve, this recommendation is not based on the likelihood that the child will have an inadequate immune response to the vaccine. Rather, the reason for deferring immunization is to avoid superimposing a reaction to the vaccine on the underlying illness or to mistakenly attribute a manifestation of the underlying illness to the vaccine.


Dennehy PH, Saracen CL, Peter G. Seroconversion rates to combined measles-mumps-rubella-varicella vaccine of children with upper respiratory tract infection. Pediatrics 1994;94:514-516.

Halsey NA, Boulos R, Mode F, et al. Response to measles vaccine in Haitian infants 6 to 12 months old: influence of maternal antibodies, malnutrition, and concurrent illness. N Engl J Med 1985;313:544-549.

King GE, Markowitz LE, Heath J, et al. Antibody response to measles-mumps-rubella vaccine of children with mild illness at the time of vaccination. JAMA 1996;275:704-707.

Ndikuyeze A, Munoz A, Stewart J, et al. Immunogenicity and safety of measles vaccine in ill African children. Int J Epidemiol 1988;17:448-455.

Ratnam S, West R, Gadag V. Measles and rubella antibody response after measles-mumps-rubella vaccination in children with afebrile upper respiratory tract infection. J Pediatr 1995;127:432-434.


Can we give vaccines by a method other than shots?

By giving vaccines as shots, the body quickly makes antibodies in the blood. Viruses and bacteria usually cause damage by first entering the bloodstream. So, the best way to fight off these infections is to make antibodies that are present in the blood. Hepatitis B, chickenpox and rabies infections can be prevented by giving a vaccine even after the person has been exposed to these viruses. This is because antibodies in the bloodstream are made more quickly after vaccination by shot than they are after exposure to the virus naturally.


Are children too young to get vaccines?

No. If children are not too young to be permanently harmed or killed by viruses or bacteria, they aren't too young to be vaccinated to prevent those diseases. Because the diseases that vaccines prevent often occur in very young infants, the only way to prevent them is to give vaccines soon after birth. Fortunately, infants given vaccines in the first few months of life are quite capable of making a protective immune response.

To watch a video clip about whether children are too young to get vaccines, go to the "Vaccines and Your Baby" video on the Educational Videos and DVDs page and select "Can babies handle vaccines so young?".


Can children manage so many different vaccines at the same time?

The mother's womb is essentially a sterile environment. The fluid surrounding the baby is free from bacteria. However, within minutes of leaving the womb, the child must confront thousands of bacteria. By the end of the first week of life, the child's skin, nose, throat and intestines are covered with tens of thousands of different bacteria.

Fortunately, from the moment of birth, infants begin to develop an active immune response to these bacteria — an immune response that prevents these bacteria from entering the bloodstream and causing harm.

The vaccines that children receive in the first two years of life are just a drop in the ocean when compared to the tens of thousands of environmental challenges that babies successfully manage every day.

To watch a video clip about getting multiple vaccines at one time, go to the video, "Vaccines: Separating Facts from Fear" on the Educational Videos and DVDs page and select "Can my baby get 4 or 5 vaccines at a time?".


Why are some vaccines given as a single dose and others as multiple doses?

Vaccines may be given multiple times for a few reasons:


Reviewed by: Paul A. Offit, MD
Date: April 2013

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.


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