By Charlotte Thålin

Dr. Thålin is the lead investigator for a study on Covid-19 immunity at the Karolinska Institute in Stockholm, where she is also an internist at Danderyd Hospital.

As a physician in a Covid-19 care unit, I celebrate the vaccines as one of medicine’s greatest triumphs. They provide extraordinary protection against severe disease and death, and are the world’s best option for returning to a more normal life. As a scientist and lead investigator for a study on Covid-19 immunity, I have also come to appreciate the significance of so-called natural immunity acquired by those who have had Covid-19, and the power of “hybrid immunity” — the protection gained when such people also get vaccinated.

While the concept of natural immunity has often been misused by people opposed to vaccine mandates, public health officials and scientists should be open to the evidence. Research, including my team’s study of the immune responses of nearly 2,150 health care workers in Sweden after infection with SARS-CoV-2 — the virus that causes Covid-19 — and vaccination, suggests that the protection gained from infection is long-lasting and that it can be significantly bolstered by a single Covid-19 vaccine dose.

These insights should be factored into vaccine policies. For example, should vaccine mandates and passports make exceptions or accommodations for people who have already had Covid-19? Should children who have been infected receive two vaccine doses when they might be well protected with one? These are just some of the questions scientists and vaccine policymakers should be asking.

Infection, like vaccination, trains the immune system to fight off disease. In both cases, antibodies are produced by what’s known as memory B cells, which help prevent future infection. Memory T cells then support antibody production and control the infection by killing infected cells.

But the immunity provided by an infection versus a vaccination differs in many ways. For example, a Covid-19 vaccine teaches the immune system to target a specific part of the virus, the spike protein, over a few hours or days. When people are infected with SARS-CoV-2 their immune system is exposed to the whole virus for several days or weeks. This provides the immune system with significant time to build a comprehensive defense if the infected person survives. These distinctions result in broader immunity for people who are infected versus people who are vaccinated.

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