A man, wearing a protective mask, rides past a propaganda banner promoting prevention against the coronavirus disease in Hanoi, Vietnam. Reuters

Lisa M. Krieger, Tribune News Service

You’re vaccinated. They’re not. When visiting, is it safe to toss the tiresome mask?

Perhaps. There’s growing evidence that vaccines not only save lives, but can stop or greatly slow spread of the COVID-19 virus — portending a day when we can see each other’s faces again. But because the findings are not yet conclusive, the impact of vaccines on viral transmission remains one of the biggest issues affecting the shape of a post-pandemic world.

Still, UCSF’s infectious disease expert Dr. Monica Gandhi is so confident of the emerging clues that an upcoming family gathering — elders vaccinated, youngsters not — will be mask-free. “I’m convinced by all the data,” she said.

For the general public, she envisions a “tiered approach” to our social lives. When everyone’s vaccinated, people can feel free to mingle with each other without restrictions, she said. If some are vaccinated while others aren’t, it’s safest to keep masking and distancing — even though the risk of infection is unlikely or very small. When no one’s vaccinated, all restrictions apply.

Why are tactics so complicated? It’s because right now, all that’s really known is that a vaccine will stop you from getting severely ill or dying.

Does it stop transmission of the virus? Remarkably, none of the vaccine trials were set up to answer this question. The focus, instead, was on the vaccine’s ability to keep people out of ICUs and morgues.

So there’s not yet definitive proof that vaccines block infection — or whether they prevent an inoculated yet infected person from passing a secret smoldering virus to others. That’s important, because people who are “silent spreaders,” never feeling sick, fuel the pandemic.

If vaccines do succeed in blocking transmission, that’s the Holy Grail, and research is ongoing to find out.

“If the vaccine proves 95% effective against asymptomatic transmission, then it is the full ‘magic bullet’ that will get us away from having to mask,” according to Dr. Warner Greene, professor of medicine at UCSF and the founding and emeritus director of Gladstone Institute of Virology and Immunology.

This is what experts say we’re learning:

Infection-fighting antibodies are found where we need them to reduce transmission: those mucous membranes that line our nose and respiratory tract, home to gobs of virus that spew out with talking, shouting and singing.

Post-vaccine, one important subtype of protective antibody, called Immunoglobulin G (IgG), is found in the blood. But now there’s new evidence that IgG is also found in the nose and throat.

There are also promising clues about a different subtype of antibody, called Immunoglobulin A (IgA). While not measured in the COVID-19 vaccine trials, it’s known to be generated by other vaccinations — so should also be triggered after our pandemic jabs. This is important because IgA thrives in the mucous-filled linings of our nose, and is deployed by the body to help fight off COVID-19 virus, once it enters our body.

“By stimulating antibodies, vaccines are likely to be able to clear the virus from your bloodstream, from your lungs and from your nasal cavities,” said Gandhi.

Remember the COVID-19 antibody therapy used to treat former President Donald Trump? That’s what vaccines do, perhaps better. The treatment — a synthetic monoclonal antibody by drugmaker Regeneron — is proven to hasten the clearance of virus from our airways.

But think of the difference between mono vs. stereo music. Regeneron’s monoclonal antibody is composed of a single IgG against one part of the virus. Vaccines, in contrast, feel like stereo. They trigger so-called polyclonal antibodies — a heterogeneous mix of IgGs against the whole virus.

In the fight against the virus, such diversity is good. “If the monoclonal antibody clears your nose, it’s reasonable to think that the vaccine’s polyclonal antibodies would also,” said Gandhi.

In a recent UK trial of the AstraZeneca — University of Oxford vaccine, participants did weekly nasal swabs at home which revealed that vaccination reduced asymptomatic infections by 49.3%, according to the journal Science. That suggests that the vaccine stymies viral spread.

When will we get results from a definitive study — confirming the presence or absence of virus by swabbing noses daily of the vaccinated and unvaccinated? That work is underway.

“Unless we answer this question, we are a masked society,” infectious disease clinician Dr. Myron Cohen of the University of North Carolina, Chapel Hill, told the journal Science. He leads the government’s COVID-19 Prevention Network. “We need to address this to become maskless.”