Before readers accuse me of being an alarmist, the headline above closely tracks the headline of the Bloomberg story by Peter Coy, How a Covid-19 Vaccine Could End Up Helping the Virus Spread. The striking bit is once you understand the medical issues Coy is raising, you’ll see his concern is legitimate. The short version is just because you got the vaccine does not necessarily mean you can’t infect others.

Until the rise of anti-vaxxers, with unseemly rush to get a Covid vaccine legitimating the vaccine refusnik (or “wait and see”) position, it was reasonable to assume that citizens would dutifully get vaccinated against dangerous contagious diseases, as much for themselves as everyone else. The high uptake rate would reduce the risk of anyone catching the disease to a very low level.

But Covid presents a very different picture. Even with enthusiastic coverage of the progress on vaccine development, a big slug of Americans still aren’t on board. A survey a few months back found only 50% would get a Covid vaccine. Even with infections and deaths on the rise, an end-of-October STAT/Harris poll found that fewer than 70% would take a vaccine with 70% efficacy:

Needless to say, this survey also shows that Americans are bad at math. Why would fewer people be on board with 90% efficacy versus 75%?

But separately, unless there’s a lot more propaganda messaging and education, the actual take-up numbers are sure to be lower if a vaccine had side effects serious enough to have decent odds of the recipient needing to take a day off from work.

And even if we assume that more people eventually become willing to get jabbed, it’s still going to take time to roll out a vaccine. The EU said they can’t vaccinate everyone before 2022. America’s access to healthcare and geographic spread means we’re pretty certain to be on a slower timetable.

And now….drumroll….here is the nub of Coy’s concern:

If everyone in the world is vaccinated, or has developed antibodies through exposure to the disease, there will be no problem. But in the early going, when only some people are protected, they could unwittingly spread the disease to people who are still vulnerable. The vaccinated people might stop wearing masks and social distancing since they aren’t themselves at risk anymore. They could be carrying the SARS-CoV-2 virus, even if they’re not getting sick from it.

How big a problem this might be is hard to say, because we don’t know for sure if immunized people are capable of shedding infectious virus. It’s possible that their antibodies will eradicate any infection pretty quickly, so they might just shed viral debris….

It’s also not yet clear how much protection the Pfizer-BioNTech vaccine and others would provide. The gold standard is to achieve sterilizing immunity, which is so strong that the virus can’t get a grip in the body at all—meaning that vaccinated people are safe to others. The human papillomavirus vaccine provides sterilizing immunity, for example. But sterilizing immunity is hard to achieve with viruses such as SARS-CoV-2, which enter through the respiratory system. The only sure way to know if the vaccine provides sterilizing immunity would be to check whether trial subjects who remain free of Covid-19 have been exposed to it, by tracing their contacts.

The Pfizer-BioNTech vaccine and others might provide just functional immunity—protecting people from the full-blown disease but not from carrying the virus. Functional immunity may also be what people get from being infected by the disease itself. They can catch it again, but will have fewer, if any, symptoms. We already know that people who are asymptomatic can spread Covid-19. In fact, that’s one of its scariest characteristics.

Coy then quotes an Australian researcher who is working on four vaccines, but not Pfizer’s. He doesn’t dispute Coy’s contention, but argues that preventing “clinical disease” will still reduce transmission, since they won’t be coughing Covid cooties. And we do know that asymptomatic cases don’t shed as much virus as symptomatic ones.

However, Coy’s point is a fair one: if a fair number of the vaccinated quit wearing masks and stop social distancing, the disease could still keep spreading: ” But if people who get vaccinated throw caution to the winds, it’s possible they could get a lot of other people sick.”