SARS-CoV-2 is a wily virus, with mysterious origins and a powerful ability to infect and spread; ‘We underestimated it.’ Via WSJ page 
Average amount of time in months that people retain immunity to each viral disease:
Spike protein
Average amount of time in months that people retain immunity to each coronavirus disease:
Bewildering complications Average amount of time in months that people retain immunity to each viral disease: