Delta variant, waning immunity reduced Pfizer vaccine’s effectiveness against household transmission

A new study by Yale School of Public Health (YSPH) researchers suggests that the Pfizer-BioNTech vaccine was more than 91% effective at reducing transmission of coronavirus in Israeli households before the emergence of the delta variant. However, the combined effect of the new variant and waning vaccine-induced immunity considerably reduced the vaccine’s effectiveness against transmission over time.

The findings are based on an analysis of medical information from 2.5 million people — about a quarter of Israel’s population — contained in a centralized database maintained by Maccabi Health Services, a state-mandated, not-for-profit health care delivery organization in Israel.

Between June 1, 2020 and July 28, 2021, nearly 1.5 million people in the dataset received two doses of the Pfizer vaccine, and only about 6,500 tested positive for coronavirus following the second dose.

Using this information, the scientists developed a household transmission model to assess the risk of infection based on an individual’s vaccination status and that of other household members. Before the emergence of the delta variant in Israel, vaccination reduced the risk of being infected by 89% and lowered the risk that people with breakthrough infections would transmit the virus to other household members by 23%. After the emergence of the delta variant, however, vaccination no longer reduced the infectiousness of breakthrough cases and provided slightly less protection against infection, the scientists found. Instead, people who had received their second dose of vaccine more than 3 months ago were only 40% less likely to be infected compared to an unvaccinated person and may have actually been more likely to transmit the delta variant to other household members if they were infected.

“Our analysis suggests that while vaccines provide good protection against coronavirus infection, this protection wanes over time,” said Virginia Pitzer, an associate professor at YSPH and the study’s senior author. “Moreover, vaccinated people who got infected with the delta variant were just as infectious as unvaccinated cases. This emphasizes the need for booster doses and for people who are infected to isolate regardless of whether or not they are vaccinated.”

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The findings are published in the journal Science.

The results mirror other studies performed in different parts of the world. In England, researchers found that both the Pfizer and AstraZeneca vaccines halved the risk of transmission from a household member with a breakthrough infection during the period when the alpha variant was dominant, but another study found vaccinated and unvaccinated cases with the delta variant infected a similar proportion of their household contacts.

Pitzer and her colleagues caution that more research is needed to fully understand the effectiveness of COVID-19 vaccines against transmission.

The researchers noted their study had some limitations. The Israeli database only contained information about when people tested positive for coronavirus and lacked key details about when exactly they were infected and how long they were infectious. Scientists therefore estimated this information using data from other studies. And the dataset did not include information on infected individuals who did not receive a COVID-19 test. The study also was conducted before the worldwide emergence of the omicron variant.

In their conclusions, the scientists said the findings provide new insight into the vaccine’s ability to not only prevent infections but to also quell spread of the virus. However, the waning of immunity and emergence of new variants make it unlikely that vaccination alone can eliminate transmission of the coronavirus, the scientists said.

Medical data for this study was provided by KSM, the research and innovation center of Maccabi Healthcare Services, led by Dr. Tal Patalon and her team.

The study was supported by funding from the National Institutes of Health, U.S. Centers for Disease Control and Prevention, COVID-19 Paycheck Protection Program and Health Care Enhancement Act, and the Pershing Square Foundation.