A year into the new pandemic, there’s still much to learn about immunity to SARS-CoV-2, the new coronavirus that causes COVID-19.

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.

Studies have shown that people who recover from COVID-19 appear to be protected for at least six months and possibly much longer. 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 are uncommon. 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.

Some research indicates that people with more severe COVID-19 may develop a stronger immune response. But people with compromised immunity—such as people receiving certain types of cancer treatment, people with untreated HIV and transplant recipients—may have a weaker response.

Vaccines provide SARS-CoV-2 immunity similar to—or better than—that of natural infection. Receiving two doses of the Pfizer/BioNTech and Moderna vaccines should provide longer-lasting protection than a single dose. The Johnson & Johnson vaccine only requires a single dose.

Most experts expect that population or herd immunity will be achieved when approximately 75% to 85% of the population as been exposed, either via natural infection or vaccination. Once this threshold is reached, the virus cannot easily spread.


Several new SARS-CoV-2 variants spread more easily than the original strain. The three authorized vaccines work well against known variants, but it’s possible that more resistant variants could emerge as the virus continues to evolve. The best way to stop the emergence of viral mutations is to ramp up vaccination as quickly as possible and to maintain precautions such as social distancing and wearing masks in the meantime.

Last Reviewed: April 29, 2021