Infection with SARS-CoV-2 initiates a myriad of innate and adaptive immune responses that while largely heterogeneous, have been shown to correlate with disease severity, progression and the development of long lasting immunity. In most individuals, these multiple immunological memory mechanisms against SASR-CoV-2 are retained for at least 8 months, explaining the low reinfection rates observed in most populations (1-10%).
Indeed, recent studies indicate previous infection provides an estimated 80-95% protection from reinfection in people under 65 years old for at least 6 months. With global estimated confirmed COVID-19 cases in excess of 180 million and more than 170 million people recovered, it would be expected that 136 million people worldwide have developed some degree of immunity to COVID-19 from a previous SARS-CoV-2 infection.
Detailed investigations of immune responses have revealed notable differences in immune mechanisms triggered by SARS-CoV-2 infection with increasing disease severity, and in response to COVID-19 mRNA vaccines.