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. Many vaccines of this type allow for life-long immunity with only one or two doses.

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 nearly been eradicated in most parts of the world.

But, there are other types of vaccines that do not involve live viruses or bacteria, which have also effected positive changes. These include:

• Inactivated vaccines: Also referred to as killed or whole-killed vaccines, these vaccines involve a dead virus or bacteria 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 well as the rabies and hepatitis A vaccines.
• Toxoid vaccines: These vaccines do not produce immunity to the actual infection. Rather, they contain a toxin made by the bacteria or virus that the immune system responds to, preventing the harmful effects of the infection. The tetanus vaccine is one such example of this.
• Subunit vaccines: These vaccines involve a protein or part of a virus or bacteria that triggers a pathogen-specific immune response. Hepatitis B, human papillomavirus (HPV), and pertussis (whooping cough) are among the diseases prevented by these types of 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:

• 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 in the United States)
• Zostavax shingles (herpes zoster) vaccine
• Yellow fever vaccine

Advantages

There are benefits afforded by 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 with killed or subunit vaccines. Because of this, booster shots are not needed as often or at all with certain live vaccines. The hepatitis A vaccine, for example, may be protective for 20 years or more.

Similarly, while most inactivated viruses require multiple doses, live virus vaccines (like the MMR and Zostavax vaccines) require only one dose. Moreover, the immune system's response to live vaccines is typically equivalent to that of a natural infection without the associated harms.

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 the cell with modified DNA to potentially treated inheritable genetic disorders.

Disadvantages

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

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

Considerations

While small, there is the risk that the weakened live virus could end up causing the illness it was meant to prevent. This is especially 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:

• People on daily high-dose steroids
• People undergoing chemotherapy
• Organ transplant recipients
• People with advanced HIV infection
• Pregnant women

Even so, there are times when the benefits of vaccination 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, whooping cough, and polio are examples of diseases that were once common but are now rarely seen. Even if eradication is not achieved, vaccines can significantly reduce the incidence and severity of many dreaded infections.

With that said, the recent anti-vaccination ("anti-vaxxing") movement threatens to reverse many of those gains with diseases like measles, declared eradicated in the United States in 2000, now re-emerging in regional outbreaks not only here but abroad.

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