The SARS-CoV-2 coronavirus pandemic now lends itself to better control in settings where there is access to fast and reliable testing and highly effective vaccination. Studies have shown that people who have recovered from a COVID-19 infection and have tested seropositive for antibodies against SARS-CoV-2 have a low risk of reinfection. However, questions remain open about the strength and duration of this protection compared to the protection afforded by vaccination.
California researchers, who reviewed numerous high-quality, large-scale studies published between now and the end of September 2021, found an 80.5%–100% reduction in the risk of reinfection for SARS-CoV-2, including its Delta variant (B.1.617.2), among those who had previously had COVID-19 but had not been vaccinated. This degree of protection is comparable to that after COVID-19 vaccination.
Thus, a study of U.S. patients showed that only 0.7% of those who had had COVID-19 experienced a recurrence of the infection. Another U.S. study revealed that 4.3% of those who had not previously been infected and 0.0% of those who had already had COVID-19 was infected. An Austrian study documented a 0.03% rate of hospitalization due to COVID-19 reinfection and a 0.01% rate of death.
In view of the strong associative evidence and biological basis in the context of protection against COVID-19, physicians should consider counseling recovered patients about the risk of reinfection and documenting the status of previous infection in medical records.
Although studies suggest that protection against reinfection with SARS-CoV-2 is strong and lasts for more than 10 months, it is not yet known exactly how long it actually lasts. Many systemic viral infections, such as measles, confer long-lasting (if not lifelong) immunity, while others, such as influenza, offer no such protection (due to the highly mutational nature of the virus). COVID-19 data collected to date are limited to short-term observations. What is encouraging is that COVID-19, even when transmitted in a mild form, still organizes a reliable antigen-specific and long-lasting humoral immune memory.
It should be understood that the presence of antibodies (and their levels) against SARS-CoV-2 does not predict with 100% accuracy the presence and extent of protection, although such misconceptions are common. After vaccination or infection, many immune mechanisms are activated in the individual, not only at the antibody level but also at the cellular immunity level.
SARS-CoV-2 infection is known to induce specific and persistent T-cell immunity that targets multiple targets (or epitopes) of the coronavirus, including its S protein. The wide variety of T-cell recognition of the virus serves to enhance protection against various variants of SARS-CoV-2, at least with respect to Alpha (B.1.1.7), Beta (B.1.351), and Gamma (P.1) variants.
For reference, people who recovered from SARS-CoV-1 infection in 2002–2003 retained memory T cells responsive to the proteins of this coronavirus even 17 years after the outbreak. Additionally, memory B cell responses to SARS-CoV-2 developed between 1.3 and 6.2 months after infection, indicating a prolonged protection.
Some people who have recovered from a COVID-19 infection may not be vaccinated against it. So, one study found that a history of COVID-19 was associated with an increase in adverse events after vaccination. There have been rare reports of serious adverse events after COVID-19 vaccination. In Switzerland, residents who can confirm their recovery from COVID-19 with a positive PCR or other tests within the past 12 months are considered equally protected as those who have had the full course of vaccination.
Although longer studies are needed, clinicians should remain optimistic about the protective effect of recovery from COVID-19. Collective immunity to combat the SARS-CoV-2 pandemic can be achieved through acquired immunity, either through disease transmission or vaccination. Acquired immunity through vaccination is certainly much safer and preferable. However, given the accumulated clinical evidence, authorities responsible for establishing and implementing COVID-19 policies should consider equating the status of vaccinated individuals with that of those who have successfully survived COVID-19 infection in terms of health requirements for attending public places, organizing work, and using public transportation.