Pfizer’s effectiveness against domestic transmission is reduced by Delta, Yale study finds

A recent study from the Yale School of Public Health examined the effectiveness of the Pfizer vaccine in preventing household transmission, using data from Israeli cases.

Aislinn Kinsella

12:34 a.m., February 03, 2022

Collaborating journalist


Sophia Zhao

A new study at the School of Public Health found that the Delta variant reduced the protection provided by the Pfizer vaccine against household transmission from breakthrough cases.

The study modeled the risk of household transmission of SARS-CoV-2 from people who took the Pfizer vaccine. The researchers compared the effectiveness of the vaccine before and after the emergence of the Delta variant, analyzing medical information from 2.5 million people in a database maintained by Maccabi Health Services in Israel. Before the Delta variant became the dominant strain, the Pfizer vaccine was found to be over 91% effective in reducing transmission. Over time, this effectiveness was reduced due to the combination of the new variant and the waning immunity from the vaccination.

“Our main findings were that the Pfizer-BioNTech vaccine was over 91% effective in reducing coronavirus transmission in Israeli households during the pre-Delta period,” Associate Professor of Epidemiology and lead author Virginia Pitzer wrote in an email. “However, the combined effect of the new Delta variant and the vaccine-induced decrease in immunity significantly reduced the vaccine’s effectiveness against transmission over time.”

The study investigated the risk of household transmission – the transmission of the virus from an infected person to another member of their household. According to Forrest Crawford, associate professor of biostatistics and co-author of the study, these household studies are useful for studying transmission between people who are in close contact with each other.

Pitzer pointed out that the high rate of secondary infection among members of the same household also makes the home environment ideal for studying virus transmission and the effects of vaccination.

“If one member of the household is infected, you can assume everyone in the house is exposed,” Pitzer wrote.

By observing who is infected within a household and studying these patterns, researchers can better understand the effectiveness of the vaccine. According to Crawford, these data allow researchers to estimate the protective effect of the vaccine, as well as its effect on reducing transmission.

According to Pitzer, these findings are consistent with studies done in other countries where different vaccines are used. She pointed to a study in England that showed vaccination reduced the risk of household transmission by 50% from an individual with a breakthrough infection when the Alpha variant, or parent variant, was dominant. A more recent study observed that vaccinated and unvaccinated people infected with the Delta variant infected a similar proportion of household members.

“We tend to see a higher number of breakthrough infections with Omicron, but we don’t yet know if vaccinated people – and particularly vaccinated and boosted people – are just as contagious as unvaccinated people if they have a breakthrough case,” Pitzer wrote.

Epidemiology professor Albert Ko agreed that vaccines are likely to be less protective against the Omicron variant than they have been against previous variants due to its many mutations. According to Ko, current data suggests that Omicron is twice as transmissible as Delta, which was already more transmissible than previous variants.

Although Ko expressed concerns about the transmissibility of Omicron, he added that the variant appears to be less virulent, meaning infections are less likely to lead to serious illness or hospitalization. He said this is especially true with vaccination and boosting, pointing out that boosted people appear to be well protected against serious illnesses despite having less protection against infections.

“If you’re vaccinated, you’re better off than if you’re unvaccinated,” Ko said. “And if you’re boosted, you’re much better off than just getting your first shot.”

Pitzer noted that the study relied on information about when individuals tested positive, but had to estimate when they contracted the virus and were infectious based on data from previous studies. The study also could not take into account people infected but never tested and identified.

Crawford pointed out that since this was an observational study, there may have been systematic differences between vaccinated and unvaccinated people that were not measured or accounted for. He stressed the need to study the transmission of the virus in various settings.

“I hope that more health systems and major governments can conduct cohort studies of infectious disease transmission in households, schools or workplaces in the future,” Crawford wrote in an e-mail. mail.

These studies, he explained, could provide researchers and public health policy makers with information about the effects of vaccination programs at the population level. For the general public, these studies could answer questions that an individual might ask about the benefits of vaccination to protect themselves or those around them.

Pitzer said the study provided new insights into the vaccine’s ability to prevent infections and limit the spread of the virus by potentially reducing the transmissibility of breakthrough infections. Although the results showed the effects of the vaccine, she noted that the combination of new variants and waning immunity makes it unlikely that transmission will be eliminated by vaccines alone.

“Our findings underscore the need for booster doses and isolating infected individuals, whether vaccinated or not,” Pitzer said.

70.1% of Israel’s population is fully vaccinated as of February 2, according to Bloomberg.

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