Studies from the early 1990s showed that individuals experimentally challenged with human coronaviruses a year after initial infection were protected against symptoms.

“there were studies done with human coronaviruses human challenge studies done with human coronaviruses in the early 1990s where they would you know someone be infected with a zero type of one of the four zero types of human coronavirus that circulates and then a year later they were experimentally challenged with that virus to see answer the question were they protected against symptoms associated with uh illness and and they were a year later but they never did those studies beyond that”