The investigators found no significant differences in reinfection rates between women and men.
As with the previous research, this study also indicates that an initial bout with SARS-CoV-2 infection appears to confer protection for at least 6 months. The researchers found no significant differences between people who were followed for 3-6 months and those followed for 7 months or longer.
Variants not included
To account for possible bias among people who got tested repeatedly, Dr. Michlmayr and colleagues performed a sensitivity analysis in a subgroup. They assessed reinfection rates among people who underwent testing frequently and routinely – nurses, doctors, social workers, and health care assistants – and found that 1.2% tested positive a second time during the second surge.
A limitation of the study was the inability to correlate symptoms with risk for reinfection. Also, the researchers did not account for SARS-CoV-2 variants, noting that “during the study period, such variants were not yet established in Denmark; although into 2021 this pattern is changing.”
Asked to speculate whether the results would be different had the study accounted for variants, Dr. Hirschwerk said, “It depends upon the variant, but certainly for the B.1.351 variant, there already has been data clearly demonstrating risk of reinfection with SARS-CoV-2 despite prior infection with the original strain of virus.”
The emergence of SARS-CoV-2 variants of concern that could escape natural and vaccine-related immunity “complicates matters further,” Rosemary J. Boyton, MBBS, and Daniel M. Altmann, PhD, both of Imperial College London, wrote in an accompanying comment in The Lancet.
“Emerging variants of concern might shift immunity below a protective margin, prompting the need for updated vaccines. Interestingly, vaccine responses even after single dose are substantially enhanced in individuals with a history of infection with SARS-CoV-2,” they added.
The current study confirms that “the hope of protective immunity through natural infections might not be within our reach, and a global vaccination program with high efficacy vaccines is the enduring solution,” Dr. Boyton and Dr. Altmann noted.