Note: I will be on vacation for a couple of weeks (yay!).  Q&A will resume on/about August 30th.  In the meantime, keep sending your questions! 

Question: What do you know about risks of reinfection within say the first or second month after a bout of Covid?

Answer: Reinfection during the short window after initial COVID is possible, but unlikely.  In fact, if a person becomes sick or tests positive within 90 days of initial infection, they are generally not classified as being reinfected.  That’s because there’s limited diagnostic availability to routinely identify reinfection, and PCR test positivity may persist (frequently up to 90 days, sometimes beyond) following initial infection without necessarily indicating live virus (PCR tests are good at detecting viral fragments).  With a few caveats, most people are considered reinfected if it has been 90+ days since their initial positive test, they have since recovered, and they are presenting with another positive test and/or a new set of symptoms.  The UK’s Office of National Statistics implements the Coronavirus Infection Survey to study this very topic.  The latest results (released end-June 2021 for the period of April 2020 through early June 2021) indicate that risk of reinfection is rare– “Overall, there were an estimated 15.2 reinfections for every 100,000 participant days at risk (95% confidence interval: 12.7 to 18.0), averaged for the entire at-risk period.”  Additionally, study authors found that the risk of reinfection “peaked around the first month after becoming at risk for reinfection and declined afterwards.” (Figure 1)  Among those who became reinfected, the median time to reinfection was 115 days.  

Three related notes: 1) Other recent research published in Clinical Infectious Diseases using university students as the study population also found low risk of reinfection– “Among 2,021 students previously infected in Fall 2020, risk of re-infection during the Spring 2021 semester was 2.2%.”  2) According to data from Public Health England, the risk of reinfection with Delta is elevated if the initial infection was 180+ days prior (see Q&A of 7/27/21). 3) Vaccination reduces the risk of reinfection (see Q&A of 8/10/21).

Figure 1. Risk of reinfection over time (UK Office of National Statistics)