We know that the vaccines now available across the world will protect their recipients from getting sick with Covid-19. But while each vaccine authorized for public use can prevent well over 50% of cases (in Pfizer-BioNTech and Moderna‘s case, more than 90%), what we don’t know is whether they’ll also curb transmission of the SARS-CoV-2 virus.

That question is answerable, though—and understanding vaccines’ effect on transmission will help determine when things can go back to whatever our new normal looks like.

The reason we don’t know if the vaccine can prevent transmission is twofold. One reason is practical. The first order of business for vaccines is preventing exposed individuals from getting sick, so that’s what the clinical trials for Covid-19 shots were designed to determine. We simply don’t have public health data to answer the question of transmission yet.

The second reason is immunological. From a scientific perspective, there are a lot of complex questions about how the vaccine generates antibodies in the body that haven’t yet been studied. Scientists are still eager to explore these immunological rabbit holes, but it could take years to reach the bottom of them.

Acting the part
Vaccines work by tricking the immune system into making antibodies before an infection comes along. Antibodies can then attack the actual virus when it enters our systems before they have a chance to replicate enough to launch a full-blown infection. But while vaccines could win an Oscar for their infectious acting job, they can’t get the body to produce antibodies exactly the same way as the real deal.

From what we know so far, Covid-19 vaccines cause the body to produce a class of antibodies called immunoglobulin G, or IgG antibodies, explains Matthew Woodruff, an immunologist at Emory University. IgG antibodies are thugs: They react swiftly to all kinds of foreign entities. They make up the majority of our antibodies, and are confined to the parts of our body that don’t have contact with the outside world, like our muscles and blood.

But to prevent Covid-19 transmission, another type of antibodies could be the more important player. The immune system that patrols your outward-facing mucosal surfaces—spaces like the nose, the throat, the lungs, and digestive tract—relies on immunoglobulin A, or IgA antibodies. And we don’t yet know how well existing vaccines incite IgA antibodies.

“Mucosal immunology is ridiculously complicated,” says Woodruff. “Rather than thinking of immune system as a way to fight off bad actors, it’s really a way for your internal environment to maintain some sort of homeostatic existence with a really dynamic outside world,” as you breathe, eat, drink, and touch your face.

People who get sick and recover from Covid-19 produce a ton of these more-specialized IgA antibodies. Because IgA antibodies occupy the same respiratory tract surfaces involved in transmitting SARS-CoV-2, we could reasonably expect that people who recover from Covid-19 aren’t spreading the virus any more. (Granted, this may also depend on how much of the virus that person was exposed to.)

But we don’t know if people who have IgG antibodies from the vaccine are stopping the virus in our respiratory tracts in the same way. And even if we did, scientists still don’t know how much of the SARS-CoV-2 virus it takes to cause a new infection. So even if we understood how well a vaccine worked to prevent a virus from replicating along the upper respiratory tract, it’d be extremely difficult to tell if that would mean a person couldn’t transmit the disease.

Making it real
Because of all that complication, it’s unlikely that immunological research alone will reveal how well vaccines can prevent Covid-19 transmission—at least, not for years. But there’s another way to tell if a vaccine can stop a person from transmitting a virus to others: community spread.

As more and more people get both doses of a Covid-19 vaccine (and wait a full two weeks after their second dose for maximum immunity to kick in), public health officials can see how fast case counts fall. It may not be a perfect indicator of whether we’re stopping the virus in its tracks—there are many other variables that can slow transmission, including lockdown measures—but for practical purposes, it’ll be good enough to help make public health decisions.

Plus, even though the data we have from clinical trials isn’t perfect, it’s a pretty good indicator that the vaccine at least stops some viral replication. “I can’t imagine how the vaccine would prevent symptomatic infection at the efficacies that [companies] reported and have no impact on transmission,” Woodruff says.

Each of the vaccines granted emergency use in western countries—Moderna, Pfizer-BioNTech, and AstraZeneca—have all shown high efficacy in phase 3 clinical trials. (The Sinopharm and Sinovac vaccines from China and the Bharat Biotech vaccine in India have also been shown to be effective at preventing Covid-19, but aren’t widely approved for use yet.)

Frustratingly, it’s just going to take more time to see if people who got the vaccine are involved in future transmission events. That’s why it’s vital that even after receiving both doses of the Covid-19 vaccine, all individuals wear masks, practice physical distancing, and wash their hands when around those who haven’t been vaccinated—just in case.