Doctor Decoded: What Are “Live Virus” Vaccines?

Live virus vaccines are deeply misunderstood.

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With more and more people curious about the vaccine development process, one type of vaccine has received a great deal of attention. Live virus vaccines, or “attenuated” vaccines, use a weakened form of the actual virus. Naturally, this sounds spooky, but the science behind it may put your mind at ease.

How Vaccines Work

The idea behind vaccines is that they help train your body to fight harmful infections. When you get a vaccine, it safely exposes your body to a small amount of the pathogen. Think of it like a fire drill, showing your body what to expect and how to respond if the “real thing” happens later.

When your immune system sees this safe amount of the pathogen, it produces antibodies. The antibodies may last for many months or years—sometimes for life—to provide immunity against the virus or bacteria. That way, if you come in contact with the pathogen later, you already have the antibodies to fight the virus off.

In some cases, when people get an infection despite having the vaccine, they tend to have less severe symptoms and a lower risk of complications. This is the case with the seasonal flu vaccine. In most cases, however, vaccines help prevent a disease altogether. The success of vaccines has helped eradicate diseases (like smallpox), or make them much less common (like measles and polio).

The Live Virus Vaccine

While all vaccines help protect against harmful infections, they do so in slightly different ways. The live virus vaccine uses a weakened, or attenuated, version of a virus. Both the measles, mumps, and rubella (MMR) vaccine and the nasal spray flu vaccine use attenuated viruses.

Naturally, some people feel nervous about getting a vaccine with a “live virus” in it. However, it’s important to understand the vaccine development process. The strength of the ideal live virus vaccine contains just enough of the pathogen to trigger antibody production, but weak enough to not cause symptoms or side effects.

Look at the history of the measles vaccine, for example. The very first measles vaccine came out in 1963. In this vaccine, the isolated measles virus went through the attenuation process 80 times. This vaccine was quite effective, but some recipients still experienced mild measles symptoms, such as fever and rash.

These mild side effects were unacceptable to the vaccine developers. Thus, the vaccine went through the attenuation process another 40 times. A new and improved vaccine came out in 1968. Thanks to this new vaccine, side effects were rare. Not only that, but measles cases plummeted in the United States. In 1963, there were nearly 500 thousand measles cases; by 1970, there were under 50 thousand cases. Learn more about the history of the measles vaccine here.

In other words, live virus vaccines (and other types of vaccines) are regularly tested, monitored, and improved. You’ve probably heard of the multi-step process of testing and approving vaccines, but the final step is ongoing evaluation. Vaccines that don’t work or consistently cause side effects are unacceptable. If a vaccine is available, it means it has a history of being safe and effective.

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