Sophia Zhao, Staff Illustrator

A recent study led by researchers at the Yale School of Public Health has indicated that regular COVID-19 boosters may be key in reducing infection.

The study — conducted by Jeffrey Townsend, professor of biostatistics at the YSPH; Alex Dornburg, an assistant professor at the University of North Carolina at Charlotte; and Hayley Hassler, a former research associate in Townsend’s lab and a current doctoral student at Georgia Tech — found that receiving a booster shot once or twice a year significantly reduces the probability of COVID-19 infection.

“The overarching goal of the study was to determine the frequencies at which regular vaccination with serially updated boosters provides substantial long-term protection from infection,” Townsend wrote. “Prevention of infection is important because infection can cause substantial morbidity, hospitalization and death.”

Despite hopes that the initial COVID-19 vaccination series would be effective at preventing long term infection, short-term studies involving SARS-CoV-2 neutralizing antibodies have indicated that antibody protection against infection wanes significantly post-vaccination and even post-booster.

For this reason, many scientists, including Ashish Jha, the White House COVID-19 Response Coordinator, are now predicting that an annually updated booster shot is a likely scenario post-COVID-19 pandemic.

“It is reasonable to expect, based on what we know about immunology and science of this virus, that these new vaccines will provide better protection against infection, better protection against transmission and ongoing and better protection against serious illness,” Jha said in a White House Press Briefing. “[B]arring those variant curveballs, for a large majority of Americans, we are moving to a point where a single annual COVID shot should provide a high degree of protection against serious illness all year. That’s an important milestone.”

However, the study coming out of the Townsend lab is one of the first ones published to observe the long-term likelihood of infection after regular boostings.

When boosted with an updated vaccine every six months, the researchers estimated that one in 10 people were projected to contract COVID-19 over a six-year period. The estimate was raised to three out of 10 people when receiving the booster shot annually. When no booster shot was received, it was estimated that nine out of 10 individuals would contract COVID-19.

“Our assessment of breakthrough infection risk given alternate boosting schedules provides essential knowledge to both personal and policy decision-making,” Hassler wrote.

“Delayed boosting is highly consequential to breakthrough infection risk and correspondingly, to the course of ongoing disease spread, prevalence, morbidity, hospitalization and mortality. Instituting regular, population-wide boosting, updated to the predominant variant, has the potential to substantially impede COVID-19.”

According to Townsend, the results of this study pertain to those with typical immune responses to the vaccines. Further studies must be conducted to assess those with atypical immune responses due to chemotherapy treatment, immunosuppressants, immune deficiencies or other conditions.

In addition to the timing of booster administrations, the researchers emphasize the importance of receiving an updated COVID-19 booster in preventing infection.

“As we have now seen with previous vaccines, we need to continue to update boosters to match the circulating strain,” Dornburg wrote to the Yale School of Public Health. “We already administer annual vaccines for influenza, and the mRNA technology would make updated vaccinations possible on an even more accelerated timescale.”

Currently, neither long-term data on COVID-19 reinfection or boosting schedules has been collected. Therefore, for this study, the researchers utilized reinfection data from closely related coronaviruses as well as the projections from short term waning antibody data for SARS-CoV-2 to estimate probabilities of infection after boosting at different intervals.

With this data, the researchers applied a comparative evolutionary framework, which estimated peak antibody responses to vaccination and inferred parameters for linear logistic regression models of infection based on these antibody levels. This allowed the researchers to estimate the probability of infection after boosting every six months, one, one and a half, two or three years.

According to data from the Center of Disease Control, or CDC, as of Jan. 2023, approximately 16 percent of the US population over five years of age has received their updated COVID-19 bivalent booster shot.


Jessica Kasamoto covers the Yale School of Public Health for the SciTech desk. She is a graduate student in computational biology and bioinformatics.