Live Vaccines and Vaccine Shedding

Vaccine Basics

Two doses of the MMR vaccine can protect you against measles
Two doses of the MMR vaccine can protect you against measles. Photo by Getty Images

Live vs. Inactivated Vaccines

Live vaccines contain a weakened or attenuated form of a virus or bacteria. They are altered so that they can't cause disease.

The other main type of vaccine is made of the inactivated virus or bacteria (whole vaccine) or just parts of the virus or bacteria (fractional vaccine).

Live vaccines are thought to better simulate natural infections and usually provide lifelong protection with one or two doses.

A second dose, like for the MMR vaccine, is given because some people don't respond to the first -- not as a booster dose. Most inactivated vaccines require multiple primary doses and boosters to get the same type of immunity.

Live Vaccines

Children have been getting live vaccines for some time. In fact, one of the very first vaccines, the smallpox vaccine, was a live-virus vaccine.

Live vaccines include:

  • MMR - the combination measles, mumps, and rubella vaccine
  • Vavivax - the varicella or chicken pox vaccine
  • Proquad - a combination of MMR and Varivax
  • rotavirus vaccines - including two oral vaccines, RotaTeq and Rotarix
  • Flumist - the nasal spray flu vaccine
  • yellow fever vaccine - an attenuated, live-virus vaccine recommended for travelers to high risk areas
  • adenovirus vaccine - a live-virus vaccine that protects against type 4 and type 7 adenovirus but which is only approved for military personnel
  • typhoid vaccine - the oral typhoid vaccine is made with a live-attenuated strain of Salmonella typhi. An inactivated, injectable version of the vaccine is also available. Either typhoid vaccine would only be given to travelers to high risk areas.
  • BCG - the bacille Calmette-Guerin tuberculosis vaccine, which is not routinely used in the United States because it mainly prevents severe TB, which isn't at all common in the United States.
  • smallpox vaccine - has not been routinely used since 1972, but is available from stockpiles if it is needed
  • oral polio vaccine - the original OPV (Sabin vaccine), which has been replaced in the United States by the inactivated polio vaccine (Salk vaccine)

Few of these live virus vaccines are actually routinely used anymore. Those that are include MMR, Varivax, rotavirus, and Flumist.

Live Vaccine Precautions

Although live vaccines don't cause disease in the people who get them because they are made with weakened viruses and bacteria, there is always a concern that someone with a severely weakened immune system could get sick after getting a live vaccine. That is why live vaccines are not given to people who are getting chemotherapy or who have severe HIV, among other conditions.

Whether or not you give a live vaccine to someone who has a problem with their immune system depends greatly on exactly what condition they have and the degree of their immunosuppression, though. For example, it is now recommended that children with HIV can get the MMR, Varivax, and rotavirus vaccines, depending on their CD4+ T-lymphocyte counts.

What about taking precautions so that you don't expose other people after your child has a live vaccine?

Vaccine Shedding and Live Vaccines

Parents sometimes have a concern about whether their healthy children should get live vaccines if they will be exposed to someone else who has a problem with their immune system, especially if they are in close contact with someone that has compromised immunity.

Fortunately, except for OPV and the smallpox vaccine, which aren't typically used anymore, children who live with someone who has an immunologic deficiency CAN and should get most vaccines in the routine childhood immunization schedule, such as MMR, Varivax, and the rotavirus vaccines. That's because it would be extremely rare for someone to contract one of these viruses from someone who got the vaccine.

A much bigger concern, actually, would be that the unvaccinated child might get a natural infection with measles or chicken pox, etc., and pass that on to the person with an immune system problem.

In fact, the latest guidelines from the Immune Deficiency Foundation state that:

Close contacts of patients with compromised immunity should not receive live oral poliovirus vaccine because they might shed the virus and infect a patient with compromised immunity. Close contacts can receive other standard vaccines because viral shedding is unlikely and these pose little risk of infection to a subject with compromised immunity.

Unless they will be in contact with someone who is severely immunosuppressed, such as getting a stem cell transplant and being in a protective environment, they can even get the live, nasal spray flu vaccine.

The concern in any of these cases is viral shedding, in which someone becomes contagious and can pass a virus to someone else. When we get sick with a cold, the flu, a cold sore, or any other contagious disease, it is not uncommon that we spread it to other people by shedding the virus or bacteria that is making us sick.

With true vaccine shedding, like with the oral polio vaccine, the vaccine virus can be shed after being vaccinated even though you didn't get sick with the virus. Fortunately, when most others are exposed to vaccine virus, they don't get sick either, as they have been exposed to the weakened vaccine strain of the virus. This was actually thought to be an advantage of the oral polio vaccine, especially in areas with poor sanitation and hygiene. Still, vaccine shedding can be a problem if the person who is exposed has a severe immune system problem.

Fortunately, vaccine shedding is not usually a problem because:

  • most vaccines are not live and don't shed, including DTaP, Tdap, flu shots, Hib, hepatitis A and B, Prevnar, IPV, and the HPV and Meningococcal vaccines
  • the oral polio vaccine is no longer used in the United States and many other countries where polio has been brought under control
  • the MMR vaccine doesn't cause shedding, except that the rubella part of the vaccine may rarely shed into breastmilk (since rubella is typically a mild infection in children, this isn't a reason to not be vaccinated if you are breastfeeding though). What about the rare case of a person developing measles after getting the MMR vaccine? In addition to being extremely rare, it would also be extremely rare for a person to transmit the vaccine virus to another person after developing measles in this way. In fact, a systemic review of the MMR vaccine in 2016 "determined that there have been no confirmed cases of human-to-human transmission of the measles vaccine virus."
  • the chicken pox vaccine doesn't cause shedding unless your child very rarely develops a vesicular rash after getting vaccinated. However, the risk is thought to be minimal and the CDC reports only 5 cases of transmission of varicella vaccine virus after immunization among over 55 million doses of vaccine. 
  • the rotavirus vaccine only causes shedding in stool, so can be avoided with routine hygiene techniques, such as good hand washing, and if immunocompromised people avoid diaper changes, etc., for at least a week after a child gets a rotavirus vaccine
  • transmission of the live, nasal spray flu vaccine has not been found in several settings, including people with HIV infection, children getting chemotherapy, and immunocompromised people in health-care settings

And of course, children shed viruses and are truly contagious if they aren't vaccinated and naturally develop any of these vaccine preventable diseases.

What You Need To Know About Live Vaccines

  • Although multiple live-virus vaccines can be given at the same time, if they aren't given at the same time, you should wait at least 4 weeks before getting another live-virus vaccine so that they don't interfere with each other.
  • It is usually recommended that children who may get a solid organ transplant get up to date on their live-virus vaccines at least 4 weeks before the transplant.
  • In addition to children getting chemotherapy, children who are getting daily steroids for 14 days or more should delay getting live vaccines for at least 3 months.
  • Live vaccines are reportedly being developed to protect against West Nile virus, RSV, Parainfluenza virus, herpes simplex, CMV, and the Dengue virus.
  • The CDC states that the yellow fever vaccine should be avoided if you are breastfeeding, but "when nursing mothers cannot avoid or postpone travel to areas endemic for yellow fever in which risk for acquisition is high, these women should be vaccinated." The precaution follows three cases of yellow fever vaccine–associated neurologic disease in exclusively breastfed babies in vaccinated mothers.
  • Vaccine shedding does not cause outbreaks, a frequent anti-vaccine myth.

If you have any concerns about your child getting a live vaccine, especially if your child or someone else at home has a problem with their immune system, be sure to talk to your pediatrician.


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Greenwood et al. A systematic review of human-to-human transmission of measles vaccine virus. Vaccine Volume 34, Issue 23, 17 May 2016, Pages 2531–2536

Lievano, FA. Lack of Evidence of Measles Virus Shedding in People with Inapparent Measles Virus Infections. The Journal of Infectious Diseases. 2004; 189 (Suppl 1) S165-70)

Mandell: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 7th ed.;

Medical Advisory Committee of the Immune Deficiency Foundation. Recommendations for live viral and bacterial vaccines in immunodeficient patients and their close contacts. The Journal of Allergy and Clinical Immunology. April 2014. Volume 133, Issue 4, Pages 961–966 .

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