• Scientists are still working to figure out how well COVID-19 vaccines prevent vaccinated people from transmitting the virus to others.
  • Health experts caution that until the majority of people are vaccinated, we should continue to wear masks in public, regardless of our vaccination status.
  • Once enough people in the community are vaccinated, the risk of transmission after vaccination becomes less of an issue.

With more than 167 million COVID-19 vaccine doses given to people in the United States — and climbing — the country is on its way to breaking free of the pandemic.

But health experts caution that until the majority of people are vaccinated, we should continue to wear masks in public, regardless of our vaccination status.

For those weary of pandemic restrictions, this nuanced messaging can be confusing.

But it’s based on what we know — and don’t know — about the effectiveness of the vaccines.

Clinical trials and real-world studies have shown that the COVID-19 vaccines are very effective at preventing severe COVID-19.

Some vaccines are also very good at preventing infections, including asymptomatic ones.

But scientists don’t fully know yet how much the vaccines reduce transmission of the virus from a vaccinated person to others.

The good news is that studies suggest that the vaccines do reduce transmission — to some extent.

Additional studies are underway now that should give us a better answer soon to the question of transmission after vaccination.

A COVID-19 vaccine that is very effective at preventing people from contracting the coronavirus in the first place can help reduce transmission. People can’t transmit the virus if they don’t have an infection.

However, the primary goal of clinical trials of COVID-19 vaccines was to show whether the vaccines prevent symptomatic infections and, in some cases, moderate or severe COVID-19.

Most trials weren’t designed to show whether the vaccines also block asymptomatic infections — those that don’t cause symptoms.

Since those first studies, researchers have carried out additional research that provides clues to how well the vaccines prevent all infections.

Last week, the Centers for Disease Control and Prevention (CDC) released initial results from a study on the real-world effectiveness of the two mRNA vaccines approved in the United States, those developed by Pfizer-BioNTech and Moderna-NIAID.

Researchers collected weekly nasal swabs from all participants to see if they had any viral genetic material, regardless of whether they had COVID-19 symptoms.

They also collected an additional nasal swab and saliva sample if people developed symptoms.

The vaccines were 90 percent effective at blocking infections — symptomatic and asymptomatic — in people who had two doses of the vaccine, and 80 percent effective in people who had one dose.

That means there was a 90 percent decrease in infections in people who were fully vaccinated compared with a similar unvaccinated group of people.

Studies like this show that the mRNA vaccines greatly reduce infections, but these are just two of the vaccines available.

While all the approved vaccines offer strong protection against severe COVID-19 and hospitalization, the Oxford-AstraZeneca and Johnson & Johnson vaccines block fewer infections than do the mRNA vaccines.

None of the vaccines are 100 percent effective at preventing infections. So even if people don’t get very sick with COVID-19, they may still contract an infection and can potentially transmit the virus to others.

Some research suggests that even if a person who has been vaccinated contracts an infection, the virus may be less infectious in this case — at least for certain vaccines.

Several research groups are measuring “viral load” — the concentration of coronavirus particles — in people who have been vaccinated.

Earlier research found that viral load is a good indicator of infectiousness.

In a study published in late March in Nature Medicine, Israeli researchers found that people who had been vaccinated with the Pfizer-BioNTech vaccine and later contracted an infection had lower viral loads than unvaccinated people who contracted an infection.

“The results show that infections occurring 12 [days] or longer after vaccination have significantly reduced viral loads at the time of testing, potentially affecting viral shedding and contagiousness as well as the severity of the disease,” the authors wrote.

Other studies have found similar results.

The Nature Medicine research was an observational study, not a randomized controlled trial, so a number of factors could have affected the results. Results may also be different for different vaccines.

In addition, while a lower viral load suggests less infectiousness, the researchers say we don’t currently know the “infectious dose” of the coronavirus for people.

Additional studies are needed, they say, to determine whether vaccines prevent transmission.

This includes studies that involve contact tracing to see if family, friends, and other close contacts of vaccinated people are indirectly protected from infection.

Researchers from the COVID-19 Prevention Network (CoVPN), which is headquartered at Seattle’s Fred Hutchinson Cancer Research Center, are currently recruiting 12,000 college students to take part in this type of study.

The study will take place at more than 20 universities across the country.

Half of the students will be randomly selected to receive the Moderna-NIAID vaccine on the day they enroll in the study. The rest will get their first shot 4 months later.

After vaccination, students will collect nasal swabs every day for 4 months to see whether they contract an infection. This test will detect both symptomatic and asymptomatic infections.

Researchers will also track infections that occur in close contacts of students in the study who contract an infection. This will allow researchers to see whether people with an infection are less likely to transmit the virus.

“The degree of transmission from vaccinated individuals will be determined by the infection rate in the close contacts,” Dr. Anthony Fauci, the head of NIAID, said last month at a White House news briefing, when announcing the study.

“We hope that within the next 5 or so months, we’ll be able to answer the very important question about whether vaccinated people get infected asymptomatically,” he said, “and if they do, do they transmit the infection to others.”

While we should soon have an answer to this question, scientists are concerned that certain coronavirus variants may reduce the effectiveness of vaccines, which could also affect transmission after vaccination.

This includes the B.1.351 variant first detected in South Africa, the P.1 variant first detected in Brazil, and the B.1.526 variant, which is spreading rapidly in New York.

All these variants contain a mutation called E484K. It may help the virus evade antibodies produced by the immune system. This might also make vaccines less effective.

Clinical trials show that the Johnson & Johnson and Novavax vaccines were less effective at preventing symptomatic infection by the B.1.351 variant compared with the original coronavirus.

Both vaccines, though, were still effective at reducing the risk of severe COVID-19.

Certain vaccines appear to work better against some variants. Pfizer recently released data showing that its vaccine was highly effective in South Africa, where the B.1.351 variant is common.

In addition, Moderna is testing an updated version of its vaccine designed to target this variant.

Going forward, this kind of COVID-19 vaccine booster shot may become common, similar to how the seasonal flu vaccines are updated each year to match the flu viruses circulating in the community.

Dr. Bob Wachter, chair of the department of medicine at the University of California, San Francisco, pointed out on Twitter that when thinking about the benefits of vaccines, there are two sets.

First, there are individual benefits, such as preventing severe illness, hospitalization, and death.

Second, there are public health benefits: Blocking transmission helps the community by preventing people from transmitting the virus to others, including people most at risk.

Once enough people in the community are vaccinated, the risk of transmission after vaccination becomes less of an issue.

Until then, there are other ways to prevent transmitting the virus to others — ones that are available to everyone.

“People should make sure they continue to do the other things that help prevent infection, such as wearing a mask, social distancing, washing hands,” said Dr. Jarod Fox, an infectious disease physician at Orlando Health.