Can vaccinated people still spread COVID-19? Here’s what we know.

We know the top vaccines from Pfizer and Moderna are pretty effective at preventing people from feeling sick. That’s great, and certainly reason enough for everyone to get one.

But while those vaccines are exceptionally good at preventing COVID-19 illness (90-95%), that’s not the same thing as preventing SARS-CoV-2 transmission.

This is the key question: Can a vaccinated person exposed to the virus pass it on to someone else?

This question is extremely important for how the next few months play out. If the coronavirus vaccine prevents transmission to a large degree — what scientists call sterilizing immunity — then those who have received both doses can live largely normal lives. A vaccinated person could visit family, friends, travel the world, all without guilt.

However, if the vaccine doesn’t prevent transmission — called effective immunity then those who have received it can still get the virus and pass it on to others. That would mean a responsible vaccinated person would still take major precautions, wary of spreading the disease before everyone (or at least most) could be vaccinated.

The answer could mean the difference between months of pandemic restrictions and economic pain. If you’re a healthcare worker, a teacher, or over 70, the answer to our question matters to you. If you own or work in a business that relies on consumer confidence (like a restaurant), the answer to our question matters to you. If you are one of the more than 100,000 Utahns who already received the first dose and are now waiting for the second one, this answer matters to you.

So here’s the short answer: We don’t know yet.

Scientists very desperately wanted to know if the Pfizer and Moderna vaccines were effective in saving lives and sickness without wasting time studying the more complicated transmission question.

But what we do have are clues to the answer, some hopeful and some concerning. Here’s what we know.

What the antibodies tell us

Let’s dig in to why there’s a difference between effective immunity and sterilizing immunity in the first place: your body’s immune system.

Scientists are still learning about our immensely complicated immune system, but one of the things we know is there are five types of antibodies with different jobs. But for the purposes of this explanation, we care about two of them: IgG and IgA antibodies.

IgG antibodies are the most common antibodies within your body, and they work really well in responding to infections and preventing you from getting sick. But IgA antibodies are the ones mostly found in the areas where your body interacts with the outside world: in your snot, saliva, tears, esophagus, etc. These IgA antibodies are constantly working to stop the bad stuff from even getting into your body.

This image will be replaced with a similar Cherrington graphic

This image will be replaced with a similar Cherrington graphic

A normal coronavirus infection triggers an immune response that’s really good at creating both of these antibodies — in fact, it creates the IgA ones first. Evidence shows that the antibodies created by natural infection aren’t perfect, but they do a relatively good job, preventing at least 75% of even minor infections in a study released Thursday.

On the other hand, a vaccine works by informing your immune system about what a virus looks like, so it can respond more quickly in the future. A vaccine is like an injection of “wanted” posters at the cellular level.

We know that the vaccine does a good job of creating IgG antibodies, but we don’t yet know if the coronavirus vaccines send wanted posters up to the IgA systems in your nose and throat. Vaccines for other diseases delivered through arm injection are a mixed bag: some create IgA antibodies and some don’t.

If IgA antibodies were created in significant numbers, we’d likely have a good amount of sterilizing immunity. Yay! If not, sterilizing immunity would be limited. Ugh.

What the animal studies say

While we don’t have enough antibody data on humans, we do have some helpful information from animal studies.

In short, we conducted two separate studies where we injected rhesus macaques with the Oxford vaccine and the Moderna vaccine. Rhesus macaques are primates that are among the species most similar to humans. They’re also not endangered in any way.

We found what we expected: The vaccines worked at preventing monkey illness when they were intentionally exposed to the virus. They didn’t cough, they didn’t have fevers, lung failure, etc.

Then we tested the macaques to see how high their viral load was in various places in their body, to see how well the vaccine worked. We found there was significantly reduced viral quantities in their lungs, where the common IgG antibodies are mostly at play. But in the nose and throat, we saw no reduction in viral load in the monkeys that had been vaccinated compared to the control group.

That implies the monkeys would still be at least somewhat contagious.

Some optimistic scientists pointed out that the high viral loads could be explained by the way the scientists infected the monkeys: essentially, they shoved a whole lot of the virus down their throats. That’s not exactly the same way humans are exposed to the disease, from droplets spread from another person. But ultimately, the results aren’t what we were hoping for.

We do have some still-in-the-works coronavirus vaccines that have shown sterilizing immunity in mice and hamsters, especially when injected up the nose. But those aren’t the ones that have made it through human or even monkey testing at this stage.

What we can learn from asymptomatic people

OK, here’s another question that might give us a clue: How well do asymptomatic people transmit the coronavirus?

We know the vaccines being studied reduce cases where people show symptoms. So if we reduce the amount of coughing, sneezing, and wheezing, how much will that slow the disease’s spread?

Remember, there’s a difference between asymptomatic and pre-symptomatic. Asymptomatic people never experience symptoms. Pre-symptomatic people don’t have symptoms yet, but they’re about to — and pre-symptomatic people are actually very contagious for the 24-48 hours before their symptoms show.

But truly asymptomatic people clearly don’t transmit the disease as frequently. Singapore’s excellent contact tracing protocols allowed them to study this, and they found that people with symptoms were 3.85 times more likely to give the disease to others than those who remained asymptomatic.

This is promising, but there might be differences between people who are asymptomatic due to their natural immune system and those who are due to the vaccine. In particular, those pesky IgA levels may well be different.

What the early vaccine data shows

Here’s the most direct clue we have: the number of vaccinated people who later become asymptomatic positives.

This actually wasn’t a goal of our human trials — all of them were checking primarily to see if the vaccine stopped people from symptomatic illness. But both Moderna and Oxford ended up giving coronavirus tests to vaccinated people at different points. Moderna nasal swabbed patients directly before the second dose, and Oxford asked some to do nasal swab tests after getting the second dose.

Made with Flourish

Moderna’s vaccine was 94% effective in stopping symptomatic cases, but that’s not all — it also was 63% effective in stopping asymptomatic cases. And remember, this is before the second dose of the vaccine was administered, which should only boost the vaccine’s effectiveness. Given the similarities between the Moderna and Pfizer vaccines, it would make sense for Pfizer’s to work in much the same way.

Meanwhile, Oxford’s vaccine was 68% effective in stopping symptomatic cases and 28% effective in stopping asymptomatic cases. That result came after the second dose: certainly not as good, but still better than nothing. Oxford’s vaccine is still awaiting U.S. approval.

While these experiments measured asymptomatic positives and not transmission directly, it would be difficult for someone to pass around the disease while they’re not testing positive through a nasal swab. It’s likely this is evidence that the vaccines have some impact on contagiousness.

Summing up the evidence

In the end, the vaccines likely limit transmission in two ways:

1. Preventing some percentage of infections totally, even asymptomatic ones. This prevents the body from replicating the virus and having enough to share with others.

2. Turning a further percentage of what would have been symptomatic infections to asymptomatic ones, thus preventing the body from coughing, sneezing, and generally shooting virus long distances.

Overall, I think it would be reasonable to estimate that vaccines reduce transmission from anywhere from 30% to 90% — a huge range, to be sure, but it depends on the vaccine used and the accuracy of these studies with limited sample size.

That result would be both good and bad. On one hand, it’s not total sterilizing immunity — someone who has been vaccinated still has a reasonably high likelihood of picking up the disease at a party and giving it to their immunocompromised friend later in the week. That means the vaccinated probably shouldn’t go to parties where there are unvaccinated people while the pandemic rages.

On the other hand, if those vaccinated are not only immune but less able to pass the virus on, that means the overall community’s transmission is going to be significantly cut. In other words, the overall number of infections will plummet.

As for whether or not you should go to restaurants, start traveling, or visiting friends once you’ve received the second dose, I think it really depends on individual factors — the vaccination rate of your community, the risk profile of the friends you’re seeing, or the place you’re traveling to being chief among them. Caution will still be warranted, but it might be more of a yellow light than a red one.

Here’s how Dr. Angela Dunn, the state epidemiologist, put it this week: “It’s just so important we still recommend individuals continue to wear face masks and physical distance as much as they can, not only because we need it to be the social norm until we reach 70 to 80% of our population being vaccinated, but also that there is a small chance that the vaccine might not be as good at preventing transmission.”

And here’s one thing we do know, as brilliantly stated by Dr. Monica Gandhi from the University of California San Francisco: Two vaccinated people can be as close as two spoons in drawer.

So whatever you do, don’t let this news stop you from getting the vaccine. Just know that while being vaccinated moves you toward the light at the end of the tunnel, it doesn’t mean you leave the tunnel completely.

Andy Larsen is a data columnist. He is also one of The Salt Lake Tribune’s Utah Jazz beat writers. You can reach him at