A booster dose of the Pfizer-BioNTech vaccine may substantially reduce the risk of infection and severe illness from Covid-19 for people ages 16 and up, according to two studies published Wednesday in the New England Journal of Medicine.

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Booster linked to lower rates of infection, illness, death

For the first study, researchers conducted a primary and secondary analysis on data from the Israel Ministry of Health from July 30 to Oct. 10 that included nearly 5 million individuals ages 16 or older who received both doses of the Pfizer vaccine at least five months earlier.

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The researchers then divided the study participants into two groups: The booster group, including all individuals who had received a booster dose of the Pfizer-BioNTech vaccine at least 12 days prior, and the non-booster group, who had not yet received a booster dose.

For the primary analysis, researchers then compared the rates of confirmed Covid-19 infections, severe illness, and death among the booster group and the non-booster group. (The researchers cited several limitations in their primary analysis, including unmeasured confounding variables, such as differences in risk-avoidance behaviors and coexisting conditions, which may have caused bias in the results.)

For their secondary analysis, researchers compared the rates in the booster group with the rates of what they called the "early postbooster group," which included individuals who had received a booster three to seven days prior. They used Poisson regression models to estimate rate ratios after adjusting for possible confounding factors.

Overall, the researchers found that the rates of confirmed Covid-19 infections and severe illness were significantly lower among the booster group than the non-booster group across all age groups included in the study. Specifically, the researchers recorded 6,160 confirmed Covid-19 infections, 175 cases of severe illness, and 35 deaths in the booster group, compared with  83,481 confirmed infections, 1,171 cases of severe illness, and 298 deaths in the non-booster group.

Second study finds similar results

For the second study, researchers analyzed EHR data from Clalit Health Services in Israel on 843,208 adults who were at least 50 years old when the study began and who had received two doses of the Pfizer-BioNTech vaccine at least five months earlier. According to the researchers, by the end of the study, 90% of participants had received a booster dose.

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Overall, participants' average age was around 69, with 60% of adults ages 65 and up. Roughly 46% of participants had hypertension, 33% had obesity, and 29% had diabetes. The analysis adjusted for sociodemographic factors and coexisting conditions.

For the analysis, the researchers compared mortality rates from Covid-19 among participants who received a booster shot during the study period (the "booster group") with mortality rates from Covid-19 among participants who did not receive the booster (the "non-booster group").

Overall, the researchers found that participants who received a booster dose at least five months after their second dose of the Pfizer-BioNTech vaccine had a 90% lower mortality rate from Covid 19-related death compared with participants who did not receive a booster dose. Specifically, the researchers recorded 65 deaths from Covid-19 in the booster group and 137 in the non-booster group.

Notably, according to the researchers, the results suggested that "socioeconomic status, diabetes, chronic obstructive pulmonary disease, ischemic heart disease, chronic heart failure, obesity, history of transient ischemic attack, and history of smoking did not have a significant association with death" from Covid-19.

Vaccination remains 'a top priority everywhere'

In an editorial accompanying the two studies, CDC's Minal Patel wrote that as of Nov. 23, boosters were available to at least some of the population in 107 countries—but she noted that population coverage with a primary series is under 40% in 105 countries and under 10% in 45 countries.

"Data provided by these two studies, along with other data regarding booster effectiveness and safety that are being generated, will provide valuable guidance for decision making in other countries, as the risk-benefit balance of introducing a booster dose is assessed," Patel said.

"Vaccination with the primary series, especially in high-risk populations, remain[s] a top priority everywhere, because this will ultimately lead to a greater reduction in severe disease and death," Patel added. (Walker, MedPage Today, 12/8)