Reinfections cleared faster than initial COVID-19 infections, including among people who never received a COVID-19 vaccine, according to a new study.
People infected with COVID-19 for a second time cleared the illness a mean of 6.6 days, compared with a mean of 9.3 days for the initial infection, U.S. and British researchers reported in the paper.
Researchers drew the findings from 1,796 first infections and 193 second infections.
The difference held for a subset of 71 people who had two well-documented infections.
Among that subset, the mean clearance time was 6.3 days, down from 9.2 days.
"Immunity from a first SARS-CoV-2 infection affects the viral kinetics of a second SARS-CoV-2 infection principally by speeding up viral clearance and thus shortening the overall time of acute infection," the researchers, including Dr. Yonatan Grad of the Harvard T.H. Chan School of Public Health, wrote.
SARS-CoV-2 is the virus that causes COVID-19.
The researchers also found that people who cleared infections quickly the first time also cleared them quickly the second time.
"While prior infection and vaccination can modulate a person’s viral kinetics in absolute terms, there may also exist some further immunological mechanism, conserved across sequential infections, that determines one’s strength of immune response against SARS-CoV-2 relative to others in the population," the researchers posited.
The analysis was based on COVID-19 tests taken from players, staff members, and affiliates of the National Basketball Association between March 11, 2020, and July 28, 2022.
Limitations include the population being primarily young, male, and healthy, though adjustments were made for age. Researchers also said that people who were infected multiple times "may differ in important immunological and behavioral ways from those who only underwent one infection during the study period."
Earlier studies have estimated strong protection from natural immunity, or prior infection, against severe illness, but that the shielding against infection waned over time.
"This research supports the textbook expectation and other papers in that our immune systems clear SARS-CoV-2 faster 2nd time," Kasper Planeta Kepp, a computational biologist who was not involved in the research, wrote on X.
Vaccine Impact
Like many U.S. businesses, the basketball association imposed harsher restrictions on unvaccinated players, leading to most receiving a vaccine. The association also imposed a vaccine mandate on workers.
Most of the people with documented infections had received a vaccine. Of the 1,796 first infections, 1,095 were among the vaccinated. Some had received a booster. Another 574 had an unknown vaccination status. Just 127 were unvaccinated.
Of the 193 people who experienced documented reinfections, 160 were vaccinated. Another 25 had an unknown vaccination status. Just eight were unvaccinated.
A person was not counted as vaccinated until 14 days had passed following their second Moderna or Pfizer shot, or 14 days after a single Johnson & Johnson shot. That counting method has been used widely because vaccine proponents argue that it takes two weeks for the vaccines to have a full impact. Critics have noted the method obscures some infections, because infections do happen in the initial days following vaccination.
The total number of people who tested, regardless of whether they tested positive, was not listed by vaccination status.
Researchers said they did not detect "significant differences in viral kinetics" when comparing the vaccinated to the unvaccinated. Kinetics include the clearance time and the peak viral concentration.
They also found that the kinetics of reinfection did not differ significantly based on the virus variant with which the person was infected. The study period covered the Alpha, Delta, and Omicron variants.
The researchers were funded by the U.S. Centers for Disease Control and Prevention, Yale University, the National Basketball Players Association, and the U.S. National Institutes of Health. Disclosures included one researcher with a consulting agreement with the National Basketball Association and Moderna, which makes a COVID-19 vaccine, and another with a consulting agreement with the league.
