As you’re hanging around out there in your mommy groups you may hear people talking about vaccine shedding. This is the idea that recently vaccinated kids become contagious and can infect susceptible individuals. Is shedding real? Yes. Is it something you need to worry about? Probably not. First of all, not all vaccines shed. Only live vaccines have the potential to shed. The live vaccines we use in the US are the rota virus vaccine, the MMR, Varicella, and nasal flu mist. Some travel vaccines are also live but they are not part of the standard childhood schedule. The older oral polio vaccine was capable of shedding as well, but this is not typically used in the US.
So does this mean that kids who have recently received a measles vaccine are causing cases of measles out there in the world? No. They’re not. We know this for a couple of reasons. First, we can tell the difference between wild measles and vaccine measles and when individuals come down with the measles, they are tested to see which strain they have. All cases of measles in the US in the last decade have been wild measles. Also, the virus in the vaccine is very very weak and while it has been identified in the stool and saliva of recently vaccinated individuals, it is in very low levels that aren’t capable of causing infection in persons with normal immune systems. There is some theoretical risk to patients on chemotherapy and those with active AIDS and these individuals are often warned that their household contacts should not receive live vaccines. However, even this risk is theoretical – there haven’t been any actual cases. Besides, think about it, how many 4 year olds do you think live with newborns? A lot, right? Are we seeing rampant cases of measles in newborns who live with 4 year olds who just got a measles booster? Because I’m not seeing it and I see a lot of babies.
Another misunderstanding that seems to be floating around is that there was this study in baboons demonstrating the presence of live pertussis bacteria in the nasal passages of vaccinated apes. What was actually demonstrated was that vaccinated animals could still become infected with wild pertussis but they didn’t exhibit symptoms. It was also demonstrated that in some cases, these individuals could spread the bacteria to unvaccinated animals even though they themselves were not symptomatic. Yes, the pertussis vaccine is not ideal. This doesn’t mean that it “doesn’t work all that well” – it works in that it protects those who have been vaccinated from getting disease. It doesn’t do a very good job of creating strong herd immunity. If your kid is not vaccinated for pertussis, they could still get it even if there is a high uptake rate of the vaccine in your community. I bring up this study because it has been misinterpreted by some to indicate that vaccinated individuals can shed pertussis and infect others. Pertussis vaccine cannot shed – it’s not alive. Below you can read the actual study and one science blogger’s very nice explanation.