The duration and strength of acquired immunity to Sars-CoV-2 amongst recovered individuals is a pressing concern. The duration of protection will not only affect the course of the pandemic, but will also impact the continued threat of COVID-19 after the pandemic has run its course.
Sars-CoV-2 has only circulated amongst humans for around 5 months. Consequently, there is no way of directly studying the duration of immunity, though with no concrete evidence of reinfection as of yet, it is safe to assume that immunity lasts at least 5 months. Our best chance at guessing how long protection lasts is to look to other human coronaviruses.
Based on studies of antibody levels after recovery from SARS (2002) and MERS (2012) coronaviruses, it has been tentatively suggested that immunity to COVID-19 might last 2-3 years. However, antibodies don’t guarantee immunity. The scales of the SARS and MERS epidemics were limited, and clear cut reinfection was never observed, meaning that protection could have been shorter-lived.
The present study instead investigated immunity and reinfection by the four seasonal coronaviruses that infect humans. The study monitored 10 subjects over a period of 35 years, and paints a bleaker picture of coronavirus immunity. Subjects were frequently reinfected by the same virus 12 months post-infection, and as reinfection relies on re-exposure, the actual duration of immunity could have been even shorter – antibody levels were found to drop substantially after just 6 months.
It is not certain that Sars-CoV-2 will behave similarly to seasonal coronaviruses, but if it does, immunity amongst recovered individuals may last less than a year. If so, knowing that an individual tested positive for COVID would be of limited use if infection occurred over a year ago. Furthermore, vaccination against COVID might need to be yearly or even half-yearly in order to maintain herd immunity and prevent ongoing transmission.
Human coronavirus reinfection dynamics: lessons for SARS-CoV-2: https://doi.org/10.1101/2020.05.11.20086439