As we’ve discussed in various contexts over recent months, a big, big question is whether mRNA vaccines prevent COVID infection itself or the just illness that the virus causes. If the vaccine keeps you from getting a severe case of the disease or dying that is obviously a huge benefit. But the initial efficacy studies could not rule out one possibility: that vaccinated individuals were still getting infected and that the vaccine was pushing their cases into the asymptomatic category. That may not be a huge difference for individuals. But it’s all the difference in the world in terms of stopping the on-going spread of the disease through the population.
Now we have a study that seems to address this question directly.
Yesterday the CDC reported the results of a study which suggest the vaccines are almost as effective against infection as they are against the disease itself.
The study followed 4,000 health care and other essential workers who got the vaccine starting in December. They tracked these individuals in 8 cities. The participants ranged from the unvaccinated, to people who had gotten just one shot, to people who were fully vaccinated (two weeks after their second shot). Rather than wait for participants to be diagnosed as they got ill or by happenstance, they tested everyone weekly. The results were dramatic. The unvaccinated group had 161 cases; the one shot group had 16 cases and the fully vaccinated group had 3 cases.
You can read the full write-up here. 63% of participants had gotten both shots by the end of the study. I had some difficulty picking apart just how many people were fully vaccinated at different times over the course of the study. In any case, the upshot of the study seems clear: the mRNA are highly protective against infections, almost as effective as they are against disease. And this was in a population facing high rates of exposure.
This seems like the kind of study that has limitations that prevent the results from being truly definitive. But the results are clear enough that it seems highly likely that people who get vaccinated have very high protection (around 90% efficacy) against any infection at all. With enough people vaccinated that should mean we can not only prevent serious disease but break the chain of transmission. If enough people get vaccinated.