After natural infection or vaccination, the immune system produces antibodies against the virus; this usually happens within a couple of weeks. But studies that only measure antibody levels don’t tell the whole story. Antibody levels in the blood normally decline over time, but the long-lived memory B cells that make antibodies remain on guard and ready to resume antibody production if they encounter the virus again. T cells, a different type of immune cell, also play a role in maintaining long-lasting protection.
People with compromised immunity—such as people receiving certain types of cancer treatment, people with untreated or advanced HIV and organ transplant recipients—may have a weaker immune response.
Studies have shown that people who recover from COVID-19 or who have been vaccinated appear to be protected for several months and possibly much longer. Receiving two doses of the Pfizer/BioNTech and Moderna vaccines provide stronger and longer-lasting protection than a single dose. The Johnson & Johnson vaccine only requires a single dose. Although antibody levels naturally decline over time, memory B cell and T cell responses continue to provide protection. Further follow-up is needed to see how long this protection lasts.
SARS-CoV-2 reinfection and breakthrough infection after vaccination can occur, but the risk is much lower compared with people who did not previously have COVID-19 or who are unvaccinated. People who are reinfected or who contract the virus after vaccination typically have milder disease. This offers real-world evidence that past infection confers protection.
Population or herd immunity occurs when enough people are immune that the virus cannot spread easily. Experts do not know the herd immunity threshold for SARS-CoV-2—or even whether it can be achieved at all.
Last Reviewed: September 7, 2021