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Robert M. Wachter is chair of the department of medicine at the University of California at San Francisco and host of the podcast “In the Bubble.”

If covid-19 has taught us anything, it’s that nothing is straightforward. We know that people who are fully vaccinated are greatly protected against infection and serious illness and are far less likely to transmit covid-19 to others. The vaccines truly are a miracle.

But here’s the bad news: Life has become even riskier for unvaccinated people, particularly those who have never had covid-19. (People with prior infections fall into a middle category, since they are at least partly protected but still require vaccination to increase the level and durability of immunity.)

The reasons that the unvaccinated are at higher risk are biological, behavioral and political.

Let’s start with the biological. The human body has not evolved to be any better at fighting the novel coronavirus, so unless our immune system is primed to fight the virus, our vulnerability remains unchanged. While treatments for covid-19 have improved somewhat since early 2020, the chances of hospitalization and death after a covid infection have not gone down much.

But while humans haven’t evolved, the virus surely has. The B.1.1.7 variant, first reported in Britain, is now the most prevalent form of the virus in the United States. This variant is far better at its job than the original coronavirus in two crucial ways: It’s about 50 percent more transmissible and, for someone who catches it, up to 60 percent more likely to be serious.

Then there’s the matter of human behavior: As more of the population is vaccinated, case rates, hospitalizations and deaths are likely to fall (although the current surges in Michigan and a few Northeast states — largely driven by infections in unvaccinated younger individuals — illustrate that improvement is not invariable). Seeing these numbers, unvaccinated people might well conclude that things have become safer and let down their guard.

The problem is that the aggregate numbers — even if they show down-trending test positivity rates, hospitalizations and deaths — may be masking an important duality. The situation may be getting enormously better in the growing vaccinated population, while at the same time growing somewhat worse in the unvaccinated group. Taken together, the overall curve shows moderate improvement. It would be like looking at a graph of lung cancer cases in a population whose rate of nonsmokers is growing. The overall curve looks good, but the risk to an individual smoker hasn’t budged. And if smokers saw the falling case rates, concluded that smoking had become safer and decided to add a pack a day, their risk would go up.

The inclination to act on sunny but partly misleading news will also influence political leaders. These officials are under tremendous pressure to open up their economies and may well see the overall improvements as reason enough to return to normalcy. Their optimistic messaging, along with the practical impact of opening settings such as bars, restaurants and gyms, will further promote virus exposure and cases in unprotected people.

The situation may not be as dire as it sounds. First, every day more unvaccinated people move into the vaccinated category. Second, as the vaccinated group grows larger, the overall impact on cases (not quite herd immunity but the same idea) makes life safer for unvaccinated people. Why? If case rates fall in a community due to more people being vaccinated, eventually unvaccinated people will be exposed to less virus, a downward pressure that may ultimately compensate for the variants’ superpowers and any uptick in risky behaviors. Finally, because the first groups to be vaccinated were those at highest risk of exposure and death, those being exposed today tend to be younger and healthier, individuals whose risk of severe disease and death is relatively low — though far from zero.

The solutions to the problem of heightened risk among unvaccinated people range from very difficult to extremely easy. Very difficult: Convince unvaccinated people that — notwithstanding the general optimism — they may, in fact, be at higher risk than before. Particularly if they’re planning to be vaccinated, now is the worst possible time to let down their guard. They should continue to wear their masks, keep their distance and avoid risky situations — even as they see their vaccinated brethren enjoying their newfound freedom. Equally challenging: Require proof of vaccination (so-called immunity passports) to access places that don’t require masks and social distancing.

Easy: Everyone gets vaccinated when their number comes up. Problem solved.

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