A new two-state study comparing coronavirus protection from prior infection versus vaccination concludes that vaccinations are still the safest way to prevent COVID-19.
The study looked at infections in New York and California last summer and fall and found that people who were vaccinated and survived a previous bout of COVID-19 had the most protection.
But unvaccinated people with past infection were in second place. By the fall, when the more contagious delta variant predominated but revaccination was not yet widespread, this group had a lower incidence than vaccinated people who were previously free of infection.
The Centers for Disease Control and Prevention, which released the study on Wednesday, noted several caveats to the study. And some outside experts were wary of the results and wary of how they might be interpreted.
“The bottom line is that you do develop some immunity from symptomatic COVID infection,” said immunologist E. John Werry of the University of Pennsylvania. “But it’s still much safer to get immunity from vaccination than from infection.”
Vaccination has long been highly recommended, even after a COVID-19 case, because both protections wear off over time — and there are too many unknowns to rely on past infection alone, especially a long-standing one, added immunologist Ali Ellebedi of the University of Washington at St. Louis.
“There are so many variables that you can’t control that you just can’t use them as a way to say, ‘Oh, I’m infected, so I’m protected,'” Ellebedi said.
The study is indeed in line with a small group of studies that have shown that unvaccinated people with a previous infection had a lower risk of being diagnosed or ill with COVID-19 than vaccinated people who had never been previously infected.
The findings of the new study make sense, says Kristin Petersen, an epidemiologist at the University of Iowa. She said a vaccine developed against an earlier form of the coronavirus is likely to become less and less effective against new, mutated versions.
However, there are a number of possible other factors, experts say, including that the effectiveness of the vaccine simply faded over time in many people, and to what extent mask-wearing and other behaviors played a role in what happened.
Another thing to consider is that the “staunchly unvaccinated” are unlikely to be tested, and only lab-confirmed cases were included in the study, Werry said.
“We may not be getting as many re-infections in the unvaccinated group,” he said.
CDC officials noted other restrictions. The study was done before the omicron variant took over and before many Americans were given booster doses that have been shown to significantly increase protection by increasing levels of virus-fighting antibodies. The analysis also did not include information on the severity of past infections or the risk of severe illness or death from COVID-19.
“The safest strategy”
The authors of the study concluded that vaccination “remains the safest strategy” to prevent infections, and “all eligible individuals should be aware of COVID-19 vaccination.”
The researchers looked at infections in California and New York, which together account for about 18% of the US population. They also looked at California’s COVID-19 hospitalizations.
Overall, about 70% of adults in each state were vaccinated; another 5% were vaccinated and had the infection. Just under 20% were not vaccinated; and approximately 5% had not been vaccinated but had had an infection in the past.
The researchers studied cases of COVID-19 from late May last year to mid-November and calculated how often each group had new infections. As time went on, protection with the vaccine alone looked less and less impressive.
By early October, compared with unvaccinated people who had not previously had an infection, incidence rates were:
– Six times lower in California and 4.5 times lower in New York among those who were vaccinated but were not previously infected.
– 29 times lower in California and 15 times lower in New York among those who were infected but never vaccinated.
– 32.5 times lower in California and 20 times lower in New York among those who were infected and vaccinated.
But the researchers found that the difference in performance between these last two groups was not statistically significant.
Hospitalization data, from California alone, followed a similar pattern.