Live Virus Vaccines: Examples, Advantages, and More

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. Examples include the oral polio, MMR, chickenpox, and smallpox vaccine.

The live vaccine relies on this weakened virus to “teach” your immune system to recognize the specific disease-causing pathogen and launch a defensive attack if it encounters it again. Many vaccines of this type give you lifelong immunity with only one or two doses.

However, there are some cases when live virus vaccines are not advised. Immunocompromised patients, people who are pregnant, and those who are breastfeeding may want to hold off on getting certain vaccines.

Examples of Live Viral Vaccines

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. Likewise, polio and measles have been declared eliminated in some parts of the world.

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

• Chickenpox (varicella) vaccines
• FluMist (intranasal flu nasal spray)
• Measles, mumps, and rubella (MMR) vaccines
• Japanese encephalitis vaccine
• Oral polio vaccine (no longer used in the United States)
• Rotavirus vaccines
• Smallpox vaccine (discontinued worldwide due to the eradication of smallpox)
• Yellow fever vaccine

How Live Attenuated Vaccines Work

An attenuated vaccine contains a weakened version of the virus or bacterium it’s meant to protect against. The version is usually too weak to cause illness, but strong enough to prompt the immune system to produce antibodies against the full virus or bacterium.

Being exposed to the virus teaches the body how to fight the disease, preventing future infection. Sometimes the immune system needs to be exposed multiple times before it can fight off an invader.

As with other vaccines, you may need more than one dose to build up your immunity enough. For some diseases, you need a second shot within weeks or months of the first one. You may also need a booster shot every year or a few years after your initial vaccination because immunity for some diseases wears off.

How Live Vaccines Differ

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. DTaP and Tdap are killed vaccines, along with the annual flu shot, polio, rabies, and hepatitis A 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, which prevents painful muscle contractions, is one such example of this type.
• Subunit vaccines: These vaccines contain 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.

Advantages

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.

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

One downside of live virus 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. These vaccine side effects can affect 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.

Who Should Not Get Live Vaccines?

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

• People on daily high-dose steroids (strong anti-inflammatory medicines)
• People undergoing chemotherapy (medicines to kill cancer cells)
• Organ transplant recipients
• People with advanced human immunodeficiency virus (HIV) infection
• Those who are pregnant

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.

Live virus vaccines are generally safe for people who are breastfeeding. The two exceptions are the smallpox and yellow fever vaccine.

Summary

Live virus vaccines contain an attenuated (weakened) form of a virus that causes disease. Examples include the MMR, chickenpox, and smallpox vaccine. Live virus vaccines are different from inactivated vaccines in which the pathogen has been killed. Many other types of vaccines exist as well.

Vaccines are one of the true public health success stories. They can reduce the number of people who get sick, the number of people who get severe disease, and, in some cases, eradicate diseases.

For protection against all types of pathogens, make sure you are up to date on your immunizations. If you’re not sure whether it’s time for a vaccine or booster, ask your healthcare provider.

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