Infection with the virus that causes COVID-19 creates a strong immune response. This is called natural immunity. If someone has had COVID and then receives a vaccine later, this is called “hybrid immunity.” Infection, like vaccination, causes the immune system to fight off disease by producing what are known as memory B and memory T cells. Research from Sweden looked at more than 2,000 health care workers who had COVID. The results showed that the protection gained from infection is long-lasting. This protection can also be significantly increased by a single vaccine dose.

Immunity by infection differs from immunity by vaccination

For example, the vaccine teaches the immune system to target a specific part of the virus, the spike protein, over a few hours or days. With an infection, the immune system is exposed to the whole virus for several days or weeks. This provides the immune system with significant time to build a complete defense. This results in broader immunity for people who are infected compared to people who are vaccinated but have not been infected. One way to think about this is that a previous infection with COVID is the same as having a single vaccination shot. Healthy people who previously had COVID also seem to have equal or higher levels of immune responses after one vaccine dose compared to people with two doses and no prior infection. However, future boosters may still be needed.

Many countries outside of the United States have recognized that prior COVID plus a vaccine (hybrid immunity) is equivalent to being fully vaccinated. In addition, it remains to be seen children who have had COVID need only one rather than two vaccine shots. If parents are unsure if their children have been previously infected, antibody tests may be useful.

More research is needed urgently, as this has important implications for public health policy, including vaccine mandates and passports.


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