Can a Vaccinated Person Still Spread the Coronavirus?


Nine vaccines have proved effective at protecting people from developing symptoms of Covid-19, the disease that can result from infection with the SARS-CoV-2 virus. Less is known about how well the various inoculations prevent people from getting an asymptomatic infection or passing the virus on to others. Preliminary evidence suggests that at least two of them do a reasonably good job protecting against any kind of infection and that one, the vaccine from Pfizer Inc. and BioNTech SE, is even more protective.

1. Why is this important?

While getting vaccinated gives people considerable insurance against falling ill with Covid, which is sometimes fatal, if they can still get silently infected with SARS-CoV-2, they might pass it on, potentially sickening people who aren’t immune. Those who are infected but never develop symptoms are responsible for 24% of transmission, one study estimated. The more SARS-CoV-2 circulates, the more opportunity the virus has to mutate in ways that enhance its ability to spread, sicken and kill people, and evade the immunity provided by existing vaccines or a past infection. Already, variants of the virus have emerged that appear to be more dangerous. Also, using vaccination to achieve so-called herd immunity, when an entire community is protected though not everyone has been immunized, requires vaccines that prevent transmission.

2. Don’t vaccines stop infection and thus transmission?

Some do and some don’t. The gold standard in vaccinology is to stop infection as well as disease — providing so-called sterilizing immunity. But it’s not always achieved. The vaccine for measles, for example, provides it; the one for hepatitis B does not.

3. Do Covid vaccines have to prevent infection to stop transmission?

Not necessarily. To the extent a vaccine prevents infection, it also prevents forward transmission. But it can do the latter without doing the former. Since SARS-CoV-2 spreads through respiratory particles from an infected person’s throat and nose, a vaccine that reduces the duration of the infection, the amount of virus in the respiratory tract (the viral load), or how often an infected person coughs may decrease the likelihood of it being transmitted to others.

4. Why don’t we know definitely how well Covid vaccines prevent infection and transmission?

The trials testing the vaccines weren’t set up to answer those questions first. Rather, they were designed to initially determine the more urgent matter of whether vaccines would prevent people from getting sick and overwhelming medical systems. To explore that question, researchers typically gave one group of volunteers the experimental vaccine and another group of equal size a placebo. After the total number of volunteers with confirmed Covid symptoms in the trial reached a pre-set level, investigators compared the number in each group to determine whether those who got the vaccine fared significantly better than those who received the placebo. For the inoculations that have worked, the vaccine groups have had anywhere from 50% to 95% fewer cases of sickness, figures that are referred to as the vaccines’ efficacy rates.

5. Why not check volunteers for asymptomatic infections as well?

That’s a more complicated undertaking since the only way to know about asymptomatic infections is to regularly test volunteers, who can number into the tens of thousands in an efficacy trial. Still, about two dozen studies involving the vaccines proven to prevent disease are doing just that.

6. What have they found?

Results so far are preliminary. The most extensive data released concern the vaccine made by AstraZeneca Plc. In a study in the U.K., volunteers are checked for SARS-CoV-2 infections using weekly self-administered nose and throat swabs. According to results as of Dec. 7, after a single dose, the group that received the vaccine had 67% fewer positive swabs than the placebo group, suggesting the vaccine cuts down on infections as well as disease. Moderna Inc. reported similar results from people who had received a single dose of its vaccine as of November. An earlier study of Moderna’s vaccine in monkeys suggested that it would reduce, if not completely prevent, onward transmission of the virus.

7. What other evidence do we have?

Since vaccines have been authorized by regulators, studies have compared outcomes for those who have and haven’t been inoculated outside of clinical trials. These studies aren’t as reliable as those in which participants are randomly assigned to a vaccine or placebo group, but when done well they offer important evidence. Notably:

  • A study published Feb. 24 of nearly 1.2 million people in Israel, which has inoculated a higher percentage of its population than any other country, estimated that the Pfizer-BioNTech vaccine was 92% effective in preventing all infections. The research team cautioned that their analysis probably couldn’t accurately capture the vaccine’s effectiveness against all asymptomatic infections given that the study subjects weren’t systematically tested for the virus.
  • An earlier analysis in Israel suggested the same vaccine cuts infection risk by 89%, although some scientists say the report’s methodology may have overestimated the effect.
  • Other data from Israel suggests that the Pfizer-BioNTech vaccine may reduce transmission even in the event that it doesn’t prevent infection. After more than 75% of people age 60 or older had received one vaccine dose and only 25% of those between the ages of 40 and 60 had, researchers from Israel’s biggest coronavirus testing lab looked at their data. For those who tested positive for SARS-CoV-2, there was a notable difference between the two age groups in the average amount of virus found in test swabs. The researchers estimated that vaccination reduces the viral load by 1.6 to 20 times in individuals who become infected despite the shot. Another study in Israel, following people who became infected after inoculation, found the vaccine reduced their viral load fourfold.
  • Initial data from the U.K., the first western country to deploy Covid vaccines, indicate the inoculation from Pfizer Inc. and BioNTech reduces the risk of infection by more than 70% after one dose and 85% after the second dose, according to Public Health England.

8. When will we know more?

As vaccination becomes more widespread, researchers should be able to discern the effect on infection and transmission patterns, although it can be difficult to distinguish the impact of inoculations from that of measures such as lockdowns and mask mandates. The completion of the vaccine trials testing for asymptomatic infections will bring additional information. Two trials are expected to finish in April. However, one is of a vaccine from China’s Sinovac Biotech Ltd., which has a reported efficacy rate as low as 50% against symptomatic disease. The other tests the Russian Gamaleya Research Institute’s shot, whose efficacy rate against symptoms was 92% in clinical trials, but it’s a small study. September should bring the completion of sizable trials of highly efficacious vaccines. Results for the shots that have proved most effective at preventing disease (95%), from Moderna and Pfizer-BioNTech, aren’t expected until October 2022 and January 2023, respectively.

Do Vaccines Prevent SARS-CoV-2 Infection?

Studies examining this question for vaccines shown to protect against disease

Source: World Health Organization’s COVID-19 candidate vaccine landscape database

The Reference Shelf

  • Related QuickTakes explore the future of SARS-CoV-2, serious vaccine reactions, Covid and kids, how the virus spreads, and masking up.
  • American epidemiologist Marc Lipsitch explains why reducing transmission is vital for herd immunity.
  • Australian epidemiologist Raina MacIntyre presents principles of vaccine programs for Covid-19 control.