Prior infection protects against variants, including omicron, study finds
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Prior SARS-CoV-2 infection protects against symptomatic reinfection with SARS-CoV-2 variants, including omicron, national data from Qatar showed.
The protection was “robust” against the alpha, beta and delta variants and lower “but still considerable” against omicron, researchers reported in The New England Journal of Medicine, citing findings from national databases that house Qatar’s PCR testing results and track vaccinations, hospitalizations and deaths from COVID-19.
The researchers used a test-negative, case-control study design “to assess the effectiveness of previous infection in preventing reinfection on the basis of a method that had recently been investigated and validated for derivation of robust estimates for such comparisons.”
They defined the effectiveness of a previous SARS-CoV-2 infection in preventing reinfection as “the proportional reduction in susceptibility to infection among persons who had recovered from infection as compared with those who had not been infected.”
The researchers considered a prior infection to be a positive PCR test that occurred at least 90 days before another positive test.
They matched SARS-CoV-2-positve patients with uninfected controls based on sex, 10-year age group, nationality and calendar date of their PCR test, and excluded anyone who received a COVID-19 vaccine. Study participants had a median age of 31 to 35 years, and the median interval between previous infection and PCR testing among case patients and controls was 279 days (interquartile range, 194-313).
Based on the data, the researchers estimated the effectiveness of previous infection in preventing reinfection to be 90.2% (95% CI, 60.2-97.6) against the alpha variant, 85.7% (95% CI, 75.8%-91.7%) against the beta variant, 92% (95% CI, 87.9%-94.7%) against the delta variant and 56% (95% CI, 50.6%-60.9%) against the omicron variant.
Effectiveness against severe, critical or fatal COVID-19 was 69.4% (95% CI, -143.6% to 96.2%) against alpha, 88.0% (95% CI, 50.7%-97.1%) against beta, 100% (95% CI, 43.3%-100%) against delta, and 87.8% (95% CI, 47.5%-97.1%) against omicron.
Among all patients with COVID-19 reinfection, severe disease occurred in only one patient with alpha, two with beta, none with delta and two with omicron. None were critical or fatal.
“Overall ... we found that the effectiveness of previous infection in preventing reinfection with the alpha, beta, and delta variants of SARS-CoV-2 was robust (at approximately 90%), findings that confirmed earlier estimates. Such protection against reinfection with the omicron variant was lower (approximately 60%) but still considerable,” the authors wrote.
“In addition,” they wrote, “the protection of previous infection against hospitalization or death caused by reinfection appeared to be robust, regardless of variant.”
The CDC continues to recommend that anyone older than age 5 years get vaccinated against COVID-19, even if they have been previously infected.
Perspective
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The value of SARS-CoV-2 infection in protecting against subsequent infection has been a significant area of debate. The CDC published a Science Brief on this topic on Oct. 29, 2021, and concluded that immunity from prior infection was high but not complete and that the level of protection was likely not the same for all viral variants. In addition, because of immunologic evidence that vaccination after infection significantly enhanced protection, CDC continued to recommend vaccination of individuals with prior COVID-19 infection. When the omicron variant was first identified, it appeared that prior infection was not offering much protection against subsequent infection with omicron but that prior infection was responsible for the lower incidence of severe disease with omicron.
The study published today in the NEJM offers further insight into the protection against omicron that previous SARS-CoV-2 infection confers. Using a test-negative design, investigators in Qatar assessed the effectiveness of previous infection in preventing reinfection and outcomes. In their population, the effectiveness of previous infection in preventing symptomatic reinfection was estimated to be 90.2% against the alpha variant, 85.7% against the beta variant, 92% against the delta variant and 56% against the omicron variant. These findings did not change after adjusting for the time from prior infection or vaccination status. In addition, among those who were reinfected, progression to severe, critical or fatal disease was uncommon, with an effectiveness of 87.8% protection against these outcomes from the omicron variant. These data are reassuring and provide evidence of protection against infection and severe disease from prior infection. However, the data also suggest that protection from prior infection is lower against infection with the omicron variant.
Despite these reassuring findings, the U.S. was swept by an extremely high number of new infections during the omicron wave, many of them among both vaccinated and/or previously infected individuals. My advice to those who have prior infection is to get vaccinated and, by doing so, achieve a much higher level of protection through “hybrid immunity.”
Perspective
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What this article says is that natural infection protects against severe disease associated with SARS-CoV-2. Not only that, but natural infection also protects against disease caused by a variety of variants, which is to say, the alpha variant, the beta variant, the delta variant and the omicron variant. So, if you're infected with one variant, you're still protected against severe disease caused by another. That's great news.
In other words, in this country, when we talk about, for example, only 63% of the population is immunized, that doesn't mean that only 63% of the population is protected. There are people who've been naturally infected, people who've been vaccinated and people who've been naturally infected and vaccinated. I think if you add that all up, you probably have about 90% of population immunity right now.
This is yet another paper that proves that natural infection protects you against severe illness.
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