获得“超级免疫”，意味着什么？

If you recently tested positive and have since recovered from COVID-19, do you now have free rein to do whatever you want? According to recent studies, prior infection vastly reduces your odds of reinfection.

While some early research has led to promising findings about post-Omicron immunity, it's important to remember that most people can't be sure which strain of SARS-CoV-2 infected them. In the United States, the Omicron variant now represents 59% of COVID-19 cases, and the Delta variant accounts for the remaining 41%, according to CDC estimates. At this juncture, there is little data on the reinfection rates of each strain. One preprint study in South Africa, however, has found that an Omicron infection may increase immunity to the Delta variant.

The study, conducted by the Africa Health Research Institute—the same researchers who first found that two doses of the Pfizer vaccine elicited a weaker immune response against Omicron—followed 13 people, 11 of whom had been infected with the Omicron variant. They found that the antibody response in people previously infected with Omicron increased protection against Delta fourfold two weeks after participants joined the study. Meanwhile, they found prior Omicron infection increased protection against Omicron reinfection 14-fold.

These findings imply Omicron could displace Delta as the dominant strain of the virus. “These results are consistent with Omicron displacing the Delta variant, since it can elicit immunity which neutralizes Delta making re-infection with Delta less likely,” the team of scientists, led by Khadija Khan, wrote in their findings.

The scientists note that if Omicron displaces Delta, and if Omicron indeed causes less severe symptoms as early findings have indicated, “the incidence of COVID-19 severe disease would be reduced and the infection may shift to become less disruptive to individuals and society.” Despite these forecasts, the World Health Organization has warned that it is too soon to assert whether Omicron is milder overall.

Another study, conducted by Oregon Health & Science University, found that a breakthrough infection in a vaccinated person leads to “super immunity." The study compared blood samples from 52 Pfizer-vaccinated employees of the university infected with different strains of the virus—Alpha, Beta, Gamma, and Delta.

The study found that antibodies measured after breakthrough cases were more abundant and more effective than antibodies generated two weeks after a second dose of the Pfizer vaccine. “You can’t get a better immune response than this,” said Fikadu Tafesse, the author of the report and an assistant professor at the OHSU School of Medicine.

“We have not examined the Omicron variant specifically," Tafesse added, "but based on the results of this study we would anticipate that breakthrough infections from the Omicron variant will generate a similarly strong immune response among vaccinated people."

Greater immunity, given the vast number of people contracting COVID-19 during the current surge, may mean the pandemic may soon ease up. “The incredible number of infections is building up population-level immunity. That’ll be crucial in terms of muting future waves,” Joshua Michaud, associate director for global health policy at the Kaiser Family Foundation, told Vox.

That being said, the Omicron variant is proving adept at evading prior infections from different variants. Preprint studies from South Africa found the risk of reinfection with Omicron during South Africa’s fourth wave was substantially higher than the risk seen during the country’s second and third waves, which were dominated by Beta and Delta, respectively. Another study from the Imperial College of London found risk of reinfection with the Omicron variant was 5.4 times greater than that of the Delta variant.