Covid-19 vaccines, now closer to becoming widely available than ever, are the ultimate tool to help tame the virus and allow people to return to some semblance of pre-pandemic life. Despite the recent good news on their progress, though, it’s unclear whether they can truly deliver on the promise of a return to normal. Why? Because as much as some vaccines have proved effective in protecting against Covid-19 symptoms, they haven’t yet shown that they can stop the virus from spreading. In fact, some vaccine makers haven’t even tested for that. And that’s a problem. Shots in development by Moderna Inc. and the partnership of Pfizer Inc. and BioNTech SE have shown stunning early success at reducing illness in infected patients. Just this week, Pfizer revealed that its vaccine was 94% effective in test subjects over 65 years old – the demographic group that needs the vaccine most. There’s still a lot left to learn, not least how protective these inoculations may be in the long run. But so far, there’s reason to believe that vaccines will induce the kind of immunity that Covid-19 survivors have developed. As for possible virus mutations, the latest vaccine technologies appear able to cope with them for now. What’s still unknown, however, is whether the vaccines also prevent transmission of the virus. Might vaccinated people who never develop Covid-19 symptoms still carry the virus and pass it to others? This is an important question, especially if many people refuse to be inoculated. Many public opinion surveys have found that a significant proportion of people are skeptical of vaccines, or at least have some misgivings. But if a vaccine does not meaningfully reduce viral transmission, it leaves the unimmunized relatively vulnerable. And if the coronavirus continues to be passed from person to person, it has a better chance to keep mutating and potentially evade our defenses. Is it possible to make sure a vaccine can block transmission? That’s a difficult thing to do completely because it requires that the shot stop the virus from replicating, even in the open spaces of the nose and throat where it’s tough for vaccines to exert their effects. It is possible, though. Logic suggests that a vaccine capable of triggering a very strong immune response would also provide so-called sterilizing immunity – that is, block transmission. Among the many vaccines now in development, Novavax Inc.’s has been found to have induced the highest antibody levels, even after taking into account the differences in the way antibodies are measured. Preclinical studies have also shown that some of the vaccines in development achieved complete viral suppression in monkeys. Johnson & Johnson, Novavax and Pfizer-BioNTech were all able to virtually eradicate the virus from animal respiratory tracts. (Vaccines in development by AstraZeneca Plc and Moderna didn’t quite manage this.) But these are not human trials, of course. Unfortunately, it’s not yet possible to find out whether vaccines are able to produce a sterilizing immunity based on current clinical trials as they have not been set up to provide that information. If participants had been asked for weekly nasal swabs, scientists could compare viral loads in vaccinated subjects and unvaccinated ones who developed Covid-19. But the Pfizer-BioNTech and Moderna trials required nasal swabs only on vaccination days and then again if subjects reported symptoms. At this point, the only way to find out whether vaccines confer sterilizing immunity is by doing another trial. Given the stakes involved, this might be warranted. Meanwhile, companies that are about to start a late-stage trial, such as Novavax, have the time and opportunity to include weekly swab tests in their protocols. It’s something they should consider. Meanwhile, where does that leave us? It’s clear we’ll all be better off with vaccines than without them, but if the virus can still spread and possibly mutate, then we can’t let our guard down even when shots are distributed. Absent widespread adoption of the vaccine, our best defense remains mask-wearing and other mitigation efforts, as well as rigorous testing and tracing regimes to better identify and stifle outbreaks.