Seroprevalence surveys suggest that, from the beginning of the pandemic to 2022, more than a third of the global population had been infected with SARS-CoV-2. As large numbers of people continue to be infected, the efficacy and duration of natural immunity, in terms of protection against SARS-CoV-2 reinfections and severe disease, are of crucial significance. The virus’s epidemiologic trajectory will be influenced by the trends in vaccine-induced and hybrid immunity.
Omicron’s immune evasion
Cases of SARS-CoV-2 reinfection are increasing around the world. According to data from the U.K. Health Security Agency, 650,000 people in England have been infected twice, and most of them were reinfected in the past 2 months. Before mid-November 2021, reinfections accounted for about 1% of reported cases, but the rate has now increased to around 10%. The reinfection risk was 16 times higher between mid-December 2021 and early January 2022. Experts believe that this spike in reinfections is related to the spread of Omicron, which overtook Delta as the dominant variant. Nonetheless, other aspects should also be considered.
Omicron’s greater propensity to spread is not unrelated to its ability to evade the body’s immune defenses. This aspect was raised in a letter recently published in the New England Journal of Medicine. The authors reported that the effectiveness of previous infection in preventing reinfection against the Alpha, Beta, and Delta variants was around 90%, but it was only 56% against Omicron.
Natural immunity
Natural immunity showed roughly similar effectiveness regarding protection against reinfection across different SARS-CoV-2 variants, with the exception of the Omicron variant. The risk of hospitalization and death was also reduced in SARS-CoV-2 reinfections versus primary infections. Observational studies indicate that natural immunity may offer equal or greater protection against SARS-CoV-2 infections, compared with immunization with two doses of an mRNA vaccine, but the data are not fully consistent.
Natural immunity seems to be relatively long-lasting. Data from Denmark and Austria show no evidence that protection against reinfections wanes after 6 months. Some investigations indicate that protection against reinfection is lowest 4-5 months after initial infection and increases thereafter, a finding that might hypothetically be explained by persistent viral shedding; that is, misclassification of prolonged SARS-CoV-2 infections as reinfections. While no comparison was made against information pertaining to unvaccinated, not previously-infected individuals, preliminary data from Israel suggest that protection from reinfection can decrease from 6 to more than 12 months after the first SARS-CoV-2 infection. Taken together, epidemiologic studies indicate that protection against reinfections by natural immunity lasts over 1 year with only moderate, if any, decline over this period. Among older individuals, immunocompromised patients, and those with certain comorbidities or exposure risk (for example, health care workers), rates of reinfection may be higher. It is plausible that reinfection risk may be a function of exposure risk.
There is accumulating evidence that reinfections may be significantly less severe than primary infections with SARS-CoV-2. Reduced clinical severity of SARS-CoV-2 reinfections naturally also makes sense from a biologic point of view, inasmuch as a previously primed immune system should be better prepared for a rechallenge with this virus.