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Multiple COVID-19 Vaccine Doses Reduce Symptoms, Length of Illness, and Need for Medical Care

Rockville, Md.  –  A study of essential and frontline workers with COVID-19 showed that two or three mRNA vaccine doses produced significant benefits. The study in the Journal of the American Medical Association found that recent vaccinations reduced fever and chills, hours of work missed, the need for medical care, and the amount of virus in the bloodstream (viral load) compared with those who were unvaccinated.

The study for the Centers for Disease Control and Prevention (CDC), conducted by a team that included Abt Global researchers, involved 1,199 participants from six U.S. states who collected nasal specimens weekly--regardless of symptoms and at the onset of symptoms—from December 14, 2020, to April 19, 2022, and tested positive for COVID-19 infection. The study found that two or three vaccine doses less than 150 days before a Delta infection and three doses before an Omicron infection were significantly associated with milder COVID-19 symptoms, while two doses before either variant were significantly associated with a lower viral load.

The study confirmed substantial differences between Delta and Omicron. While only 3.9 percent of Delta infections were asymptomatic, 20.2 percent of Omicron infections were. If people didn’t know they had Omicron, they could have been unwitting spreaders and contributed to Omicron’s higher transmission rate.

The report noted that the precision and statistical significance of some estimates varied and could have been affected by “confounders,” that is, non-medical factors.  For example, different employer isolation protocols could affect the number of work hours missed.

“These factors are a growing issue for this kind of research as people’s behavior changes, especially when vaccinated and unvaccinated people also behave differently in other ways,” says Abt Health Economics Principal Associate Lauren Olsho, a co-author of the article. “Our study suggests staying up to date with your vaccinations and boosters can help keep you from getting as sick. But especially if we do see a big winter surge, additional precautions like masking can still be another layer of protection even if you are vacinated.”

“It’s hard to say what will happen from here as masking becomes less common across the board. Everyone wants to get back to normal even though hundreds of people still die from COVID-19 in the U.S. every day,” she added. “COVID-19 is still very much with us.” 

About Abt Global
Abt Global is a global consulting and research firm that combines data and bold thinking to improve the quality of people's lives. We partner with clients and communities to advance equity and innovation—from creating scalable digital solutions and combatting infectious disease, to mitigating climate change and evaluating programs for measurable social impact.

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