Active-duty service members who received a COVID-19 vaccine booster shot were significantly less likely to be infected and show symptoms of the disease during the surge of the Omicron variant this winter, according to a recent study. 

The study highlights the effectiveness of booster shots, which were first formally recommended by the Centers for Disease Control and Prevention in November. 

“The findings support the policy of booster doses at five months after the primary series and show improved vaccine effectiveness with a booster dose even during times of a newly emerging SARS-CoV-2 variant,” said Shauna Stahlman. She is a senior epidemiologist in the Epidemiology and Analysis Branch of the Armed Forces Health Surveillance Division. 

But a separate study also found that the number of active-duty service members who were voluntarily getting booster shots lagged behind the general U.S. population. 

As of Jan. 31, only 24% of active-duty service members who were eligible for a booster had voluntarily received the additional shot, according to Army Col. (Dr.) James Mancuso. Mancuso chairs the Preventive Medicine and Biostatistics Department at the Uniformed Services University of Health Sciences’ Hebert School of Medicine. His group led the booster uptake study in collaboration with Stahlman and AFHSD. 

By comparison, in the general U.S. population as of Feb. 10, about 46.8% of the booster-eligible population had received the additional shot. “This is likely reflective of [active-duty service members] being a relatively younger and healthier population,” Stahlman said. 

The study found that service members were less likely to get a booster shot if they were younger, lower in rank or had lower education levels. Similar studies of the civilian population show that younger and healthier people have been less likely to seek out vaccines and booster shots. 

Overall, the rate at which service members sought out and received booster shots increased during the surge of infections linked to the Omicron variant beginning in December 2021. Vaccine uptake among active-duty service members also went up in November 2021 after the CDC recommended that all adults get a booster, Stahlman noted. 

The Pentagon mandated initial COVID-19 vaccinations for service members in August 2021. However, booster shots remain voluntary. 

Booster Mandate Coming? 

Stahlman said the study’s findings suggest that “high levels of booster uptake among [active-duty service members] are unlikely to be accomplished on a voluntary basis.” 

In mid-December, the Pentagon said there were “active discussions” within the DOD about making the booster shots mandatory for service members. The Defense Department has not issued any requirement for a booster because the booster doses are only used under emergency authorization. The primary vaccination series (e.g., generally the first two shots) are fully approved by the Food and Drug Administration. 

Mancuso said that currently, the best way to promote booster vaccination is to encourage service members to discuss their own personal risks of COVID-19 and benefits from getting the booster shot with their primary unit-level health care providers.  

“These providers should be encouraging vaccination due to its benefits to the health of the individual as well as to military readiness and overall the health of the force.” 

FDA’s vaccine advisory committee met April 6 on questions about booster doses. The CDC has not yet scheduled a meeting on the subject of boosters with its advisory group on vaccines. 

Who Didn’t Get Boosted 

The study found that active-duty service members were less likely to get a booster shot if they had a previous COVID-19 infection. Active-duty service members in the U.S. received fewer boosters compared to those stationed overseas. 

Other factors that appeared to result in lower booster uptake were being male and serving in the Marine Corps. 

The study did not find any significant differences in booster shot uptake among different races and ethnicities, as was seen in a previous study of the primary vaccination series. 

Booster Vaccine Effectiveness 

Vaccine effectiveness “was significantly higher” among service members who received a booster shot compared to those who only received the initial vaccination, regardless of the month or time since their primary vaccination, Stahlman said. 

Even during the surge, service members who received a booster shot had 43% to 63% reduced odds of developing symptomatic COVID-19 when compared to those who were vaccinated with the primary series but didn’t receive the booster, according to the study. 

“Our patterns of vaccine effectiveness estimates are generally similar to those observed in the U.S. population and among military veterans,” she added. 

The booster vaccine effectiveness study reviewed military vaccination data for active-duty service members who were tested for SARS-CoV-2 between July 2021 and January 2022. The study evaluated the three existing vaccines from Pfizer/BioNTech, Moderna, and Johnson & Johnson/Janssen.