With Israel having one of the highest vaccination rates, this study is so far the largest real-world observational study to compare natural to vaccine-induced immunity. Data came from the Maccabi Healthcare Services database with records on 2.5 million Israelis. People who were vaccinated in January and February but had never been infected with SARS-CoV-2 were found to be 6 to 13 times more susceptible to infection by June, July, and August than the unvaccinated who'd had an infection. Fortunately, in this analysis, overall infection rates in the vaccinated group and re-infection rates in the previously infected group were low – 238 infections with SARS-CoV-2 out of 16,000 people in the vaccinated group compared to 19 re-infections out of around 16,000 people in the unvaccinated group that had already had an infection.
Comparing previously infected people who remained unvaccinated with previously infected people who received one dose of the Pfizer-BioNTech vaccine, the unvaccinated group was two times more likely to be reinfected.
Limitations of this study include its retrospective analysis that doesn't allow for controlling of retesting. For instance, previously infected people may not have gotten retested if their symptoms were mild and they thought they were immune, which could change the re-infection rate found in the study.
Another COVID-19 researcher, Michel Nussenzweig at Rockefeller University, has data showing that people who have naturally-acquired immunity from COVID-19 produce increasing numbers and types of COVID antibodies for up to one year after infection, while vaccine-induced immunity results in a less potent and less broad antibody response several months after the second dose of vaccine.
These findings raise new questions concerning recent vaccine mandates, such as should SARS-CoV-2 survivors be required to be vaccinated and is more than one dose of vaccine necessary in this group of people?
Although rare, re-infection with SARS-CoV-2 can be severe. One thing is certain: Infection prevention is the best strategy, especially considering the potential for chronic complications after SARS-CoV-2 infection. Known as long COVID, or post-acute COVID-19 syndrome, symptoms last for four or more weeks after SARS-CoV-2 infection and include shortness of breath, fatigue, poor endurance, cognitive impairment, cough, chest pain, headache, heart palpitations, muscle and joint aches, abdominal pain, diarrhea, insomnia, fever, lightheadedness, mood changes, rash, loss of taste and smell, and irregular menstrual cycles. Development of chronic autoimmune conditions that cause organ damage can also occur after the original infection has cleared.