Natural immunity doesn't always work this well. There are plenty of diseases where protection wanes after six months, like RSV, or Norovirus, and many common cold coronaviruses.

Natural Immunity to COVID-19 Detected at 20 Months After Infection: Study

 Zachary Stieber, Epoch Times

Protection against the virus that causes COVID-19 among the recovered was detected by researchers at 20 months post-infection, adding to the body of evidence that such protection, known as natural immunity, is long-lasting.

Researchers found antibodies against the SARS-CoV-2 spike protein receptor-binding domain (RBD) in 99 percent of study participants who tested positive for COVID-19, with some having had the illness as long as 20 months prior.

"The major takeaway is that natural immunity … is strong and durable," [Dr Dorry] Segev told The Epoch Times in an email.

Dr. Dorry Segev, the director of the Epidemiology Research Group in Organ Transplantation at Johns Hopkins University.

There were three groups involved in the study. Of the 295 people who "thought they had had Covid" about half of them actually had. And of 246  who thought they "had never had Covid" 10% appear to have had an asymptomatic infection.