Live virus vaccine vs. killed virus vaccine: What's the difference?

stockphotopro_16374kcu_illustration_of_aIf you’ve been reading up on vaccines lately (and it seems like everyone has), you’ve probably read that there’s a difference between the injectable and mist versions of vaccines. And if you’re like me, you might not know what that difference is. Well, here it is: the injectable version is made with “killed virus” and the mist version is made with “live virus.” If, like me, you’re still confused, here’s some help from Ken McIntosh, MD, an infectious disease and vaccine specialist at Children’s Hospital Boston.

The killed vaccine has been used for decades, and during that time has improved. The improvements are that it has become more potent and, most important, contains less of the possible non-influenza components that were in the earliest killed vaccines. These were (mostly) bits of egg protein coming from the embryonated eggs that were used to produce the vaccine. Those non-viral bits were responsible for most (although not all) the bad reactions (fever, headache, sore arms) that occurred in the old days. Now someone can expect a somewhat sore arm or nothing at all. Fevers and headaches are quite rare.

The live vaccine is newer and has been licensed only for a few years. The idea, however, has been around for a very long time. It contains an attenuated virus (that is, it has been altered so it produces natural immunity but does not produce the disease, influenza), and causes a weakened infection that lasts for a few days to a week or more, depending on how much immunity the recipient has. Side effects are occasional symptoms that resemble, but are much milder than, normal influenza (runny nose, fever, sore throat, cough and rarely others), and last for only one or two days. Because it is a live virus, it’s not recommended for people who have a list of underlying diseases, and so far it is only recommended for people between 2 and 49 years of age. Also, because it still has some (although VERY little) ability to spread, it is not recommended if there is someone with a weakened immune system in the household.

The information we have about these two vaccines for seasonal flu is that the live vaccine gives somewhat better protection than the killed vaccine in smaller children. In adults there flu_logo 210pxare some conflicting data, but some people think the killed vaccine works better. Just where the dividing line is (which age, that is), I don’t know.  But the important information is that both types of seasonal vaccine work, and both, if used properly, are safe.

Regarding the two vaccines for the 2009 H1N1 strain, we can only speculate, because neither has been tested for efficacy. But both vaccines have been produced in the same way as the seasonal flu vaccines, so it’s a good bet that they will behave the same way.