A study of essential and frontline workers with COVID-19 showed that two or three mRNA vaccine doses reduced severity of COVID-19 symptoms. 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 nose (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. In addition, those who are vaccinated may be more likely to report symptoms than those who aren’t vaccinated. The findings suggest that staying up to date with COVID-19 vaccinations and boosters can help keep people from getting seriously ill.