Sophia Zhao

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

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

“Our main findings were that the Pfizer-BioNTech vaccine was more than 91% effective at reducing transmission of coronavirus in Israeli households in the pre-Delta period,” associate professor of Epidemiology and senior author Virginia Pitzer wrote in an email. “However, the combined effect of the new Delta variant and waning vaccine-induced immunity considerably 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 individual to another member of their household. According to Forrest Crawford, associate professor of biostatistics and co-author on the study, these household studies are useful to study the transmission among 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 household settings ideal for studying virus transmission and the effects of vaccination. 

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

By observing who gets infected within a household and studying these patterns, researchers can gain insight into the vaccine’s effectiveness. According to Crawford, this data allows researchers to estimate the protective effect of the vaccine, as well as its effect on reducing transmission.

According to Pitzer, these findings are in accordance with studies performed in other countries where different vaccines are used. She pointed to a study in England that showed that vaccination provided a 50 percent decreased risk of household transmission from an individual with a breakthrough infection when the Alpha variant, or the original 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 know yet whether vaccinated people –– and particularly vaccinated and boosted people –– are just as infectious 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 seems to be less virulent, meaning that infections are less likely to result in severe illness or hospitalization. He said that this is especially true with vaccination and boosting, emphasizing that boosted people appear to be well protected against severe illness despite having less protection against infection. 

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

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 account for individuals who were 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 emphasized the need for studying virus transmission in a variety of settings.

“I hope more major healthcare systems and governments can conduct household, school, or workplace cohort studies of infectious disease transmission in the future,” Crawford wrote in an email.

These studies, he explained, could provide researchers and public health policymakers with information on the effects of vaccination programs at the population level. For the general public, these studies could answer questions an individual might have about the benefits of vaccination for protecting themself or those around them.

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

“Our results emphasize the need for booster doses and for people who are infected to isolate regardless of whether or not they are vaccinated,” Pitzer said.

70.1 percent of the population of Israel are fully vaccinated as of Feb. 2 according to Bloomberg.