The newly available vaccines have been criticized for claiming >90% effectiveness in preventing COVID-19 as judged by people developing two or more symptoms as the criteria to get tested for confirmation of illness. So the critics say, this leaves the possibility that up to 20% of people might still get infected, but remain asymptomatic. But wouldn’t those people very likely have benefitted nonetheless? Isn’t preventing symptoms what is most needed? And wouldn’t a mild, asymptomatic infection serve to further bolster their immunity and be a gain rather than a loss?

Yours is a correct analysis on all counts. The name of the game in fighting infection is preventing frank illness by limiting symptoms and the possibility of complications and mortality for serious illnesses when people are compromised by having a week immune system or other chronic illness. As you know, even what in most people would be a mild case of the flu may well be lethal, with even colds predisposing to lethal pneumonia in such individuals. So this is why precautions need to be taken for those known to be at greatest risk from COVID-19 infection. When people are exposed but only develop mild symptoms, that is a clear win for whatever brings about that circumstance and, in fact, that is one consequence in the spectrum of possibilities among a group of people treated with the current vaccines—it is a numbers game. The degree of immunity in response to the vaccination will prevent symptoms with a particular viral load up until a maximum containment level is reached. Beyond that, an overwhelming onslaught of infectious viral particles will, in fact, swamp the immune system and result in symptomatic illness despite the vaccinated status. So what typically happens, is an effective vaccination is adequate to the task of preventing onset of symptoms with a typical viral exposure experienced by people going about their normal affairs and encountering people who are infectious, or who visit a location where infected people have been present for enough time to shed particles into the atmosphere. So people with a modest increase in their resistance from a single vaccination have a good chance of remaining symptom-free. Even if they are exposed to the virus and have a quite mild ramping up of viral replication until the immune system effectively stamps it out, that is clearly a win, and again a victory as a result of having been vaccinated that otherwise would, in many cases, cause frank symptoms that could even worsen and become quite serious. So that is, in fact, what will happen. The vaccination shifts vulnerability to greatly lessen the consequences for a given viral exposure. And anything below the symptomatic threshold will, in fact, continue to bolster immunity as a consequence of the exposure. This is not to say it is desirable to have a subclinical exposure to pathogens, as there is always some tissue injury that happens. But if the strategy of obtaining a vaccination reduces the likelihood of overt symptomatic illness, that is clearly desirable, even though some may still have an asymptomatic subclinical bout with the virus despite the immune system emerging victorious by containing it. So we see this as a weak argument and a faulty one in discouraging what can be a helpful measure.