1. Why is this important?

While getting vaccinated gives people considerable insurance against falling ill with Covid, which is sometimes fatal, it’s so far no assurance that they won’t get silently infected with SARS-CoV-2 and 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 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. Earlier, Moderna Inc. reported similar results from people who had received a single dose of its vaccine as of November.

7. What other evidence do we have?

Animal experiments provide clues. Notably, a study of Moderna’s Covid vaccine in monkeys suggested that it will reduce, if not completely prevent, onward transmission of the virus. We are also gathering data during the rollout of vaccines.

• 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.

• An analysis in Israel, which has inoculated a higher percentage of its population than any other country, suggests 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 if it doesn’t protect against 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.

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.

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