What You Need to Know About Live Virus Vaccines

A live virus vaccine contains a live, weakened (attenuated) virus that helps your body develop an immune response without you developing symptoms of the disease it is intended to prevent.

The weakened virus "teaches" your immune system to recognize the disease-causing pathogen and launch a pathogen-specific attack if it encounters it again. Many vaccines of this type give you lifelong immunity with only one or two doses.

MMR vaccine

How Live Vaccines Differ

Live attenuated vaccines have been among the most successful preventive interventions in medical history. Due to global vaccination programs, smallpox was declared eradicated in 1980, while polio and measles have been declared eliminated in some parts of the world.

Live viral vaccines are just one type of vaccine. There are other types that do not involve live viruses or bacteria. These include:

  • Inactivated vaccines: Also referred to as "killed" or "inactivated whole-cell" vaccines, these involve a dead virus or bacterium that the immune system still regards as harmful, triggering a pathogen-specific immune response. The annual flu shot is one such example of this, as are the rabies (spread through saliva of animals) and hepatitis A (liver infection) vaccines.
  • Toxoid vaccines: These vaccines do not produce immunity to the actual infection. Rather, they contain a chemically altered toxin made by the bacterium or virus that the immune system responds to, preventing the harmful effects of the infection. The tetanus vaccine (prevents painful muscle contractions) is one such example of this type.
  • Subunit vaccines: These vaccines involve a protein or part of a virus or bacterium that triggers a pathogen-specific immune response. Hepatitis B (liver infection), human papillomavirus (HPV, a sexually transmitted infection that produces warts), and pertussis (whooping cough, a respiratory infection) are among the diseases prevented by these types of vaccines.
  • Conjugate vaccines: This type of vaccine targets bacteria that have an outer coating made up of sugar molecules (polysaccharides). This coating is designed to disguise the bacteria and prevent an immune response. These vaccines are effective in young children who have an immature immune system. Haemophilus influenzae type B (Hib) vaccine (prevents a severe infection) is one such example.
  • Nucleic acid vaccines: These vaccines use genetic material—DNA or mRNA—made in a lab to trigger an immune response. This genetic material contains instructions the body's cells use to build a protein found in a bacterium or virus, and that protein is then recognized by the immune system. The COVID-19 vaccines from Pfizer and Moderna are both examples of mRNA vaccines.

Types of Live Viral Vaccines

Live attenuated viral vaccines are typically delivered by injection, although some are available as a nasal spray or taken by mouth. These include:

There are also live attenuated vaccines used to prevent bacterial infections like cholera (bacterial disease of the small intestine), typhoid (spreads through contaminated food), and tuberculosis (serious lung infection).


There are benefits of using live attenuated vaccines that other vaccines may not deliver.

By and large, live vaccines are more durable, meaning that the body will retain its immune "memory" of a pathogen longer than it will with killed or subunit vaccines. Because of this, booster shots are not needed as often or at all with certain live vaccines. The live attenuated hepatitis A vaccine, for example, may protect you for 20 years or more.

Moreover, your immune system's response to live vaccines is typically equivalent to that of a natural infection without the associated harms. Most live vaccines can give you a lifetime of protection.

The need for multiple shots and booster shots—and people not returning to get them—is one of the reasons why immunization rates are often lower from inactivated vaccines compared to live ones.

Live virus vaccines are also being increasingly used by researchers as "viral vectors" for gene therapy. Because the weakened viruses are still "programmed" to attack specific cells, they can latch onto and implant a cell with modified DNA to potentially treat inheritable genetic disorders.

Additional research on using vaccines in this way is needed.

Disadvantages and Considerations

One downside of live vaccines is that they require refrigeration, which can limit the ability to use them in resource-limited areas. If these vaccines are not kept at the correct temperature, they can become inactive. If this happens, immunization may not be effective.

Also, some live virus vaccines come as a powder and require reconstitution (restoring from a dry state) with a specific diluent (fluid) before they can be administered. Administrator error can also undermine the effectiveness of the vaccine.

Although small, there is the risk that the weakened live virus could end up causing the illness it was meant to prevent. This is especially the case in people who are severely immunocompromised. In the absence of a normal immune response, the weakened virus has the potential to replicate and, in rare cases, reverse to its natural virulent state.

As a general rule, live attenuated vaccines should not be given to severely immunocompromised people, including:

Even so, there are times when the benefits of vaccination in these individuals outweigh the risks. In such cases, expert consultation is needed to determine whether the use of a live attenuated vaccine is appropriate.

A Word From Verywell

Vaccines are one of the true public health success stories. Tetanus, diphtheria, mumps, and polio are examples of diseases that were once common but are now rarely seen in the United States. Even if eradication is not achieved, vaccines can significantly reduce the incidence and severity of many dread infections.

With that said, the recent anti-vaccination movement threatens to reverse many of those gains with diseases like measles. This highly contagious viral disease was declared eliminated in the United States in 2000, but now is reemerging in regional outbreaks not only in America but abroad.

Frequently Asked Questions

  • Does the COVID-19 vaccine contain a live virus?

    No. The vaccines used in the United States include two mRNA vaccines (Pfizer and Moderna), one viral vector vaccine (Johnson & Johnson), and one subunit vaccine (Novavax). These shots contain a gene from the COVID-19 virus, but not the whole live virus.

  • How does a live attenuated vaccine work?

    An attenuated virus contains a weakened version of the virus or bacteria it’s meant to protect against. The version is usually too weak to make the person ill with that disease, but strong enough to prompt the immune system to produce T-lymphocytes and antibodies against the full virus or bacteria. This teaches the body how to fight the disease, preventing future infection.

  • Why do I need to get more than one dose of a vaccine?

    For some diseases, one dose of a vaccine doesn’t build up your immunity enough. You need a second shot to protect you from the disease. Sometimes, you need a booster every year or a few years after your initial vaccination because immunity for some diseases wears off.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Pat Bass, MD
Dr. Bass is a board-certified internist, pediatrician, and a Fellow of the American Academy of Pediatrics and the American College of Physicians.