A year after discovering the SARS-CoV-2 virus, the pandemic’s end seems just beyond the horizon as more and more hold out hope that a highly effective vaccine may soon be widely available. Returning to a Covid-free reality is a yearning we all share, but there’s reason to believe we may never be Covid-free again because of the genuine possibility that Covid-19 will become an endemic disease; people once infected may become reinfected.

We still know little about Covid-19 reinfection incidence. There have been official cases across continents and undoubtedly many more that have gone unreported. A Lancet report confirmed that in mid-October, the virus reinfected a man in Nevada. His symptoms were more severe during the second infection, but he has since recovered. Whether symptoms are worse on the first or second infection seems to vary from case to case. 

That means reinfection occurs despite the body’s immune response with antibody or T-cell production. Common knowledge suggests that when I am sick, my body mounts an immune response against my specific disease to prevent sickness down the road. While this is technically correct, a vigorous immune response, including antiviral antibodies and T-cells, does not guarantee lasting protection from disease. While some antibodies last decades, such as measles, others may last six months or less. We do not know precisely how long our protective immune response to Covid-19 may last, but early indications seem to be leaning towards just a few months.

This could explain why confirmed cases of reinfection are so rare, as well as why many more reinfections probably occur. If the first or second infection is asymptomatic, that infection may go undetected by the host. Of the fifty-five million global infections to date, it seems likely that Covid-19 reinfected more than just five people. 

Because of these uncertainties and the importance of reinfection, we call for a significant research effort to investigate this topic. This study would be constructed by first sequencing the virus of a large cohort of Covid-19 patients when they first present. Then we would follow them and regularly test with rapid antigen tests. These tests are the most efficient method of tracking those most contagious: asymptomatic and presymptomatic patients. Using rapid tests means the bulk of this study could include asymptomatic and presymptomatic patients, whose sequences could be used for later research.

For every patient who tested negative and then later tested positive, we resequence to determine one of two possibilities. Either the virus persisted and reemerged, or it was a new infection. The true magnitude of reinfection may only be recognized after two or three years of such studies.

Understanding reinfection is necessary to inform Covid-19 control strategy moving forward. Herd immunity strategists claim the potential to rapidly end the pandemic and save the economy (at the expense of millions dead). They believe a population exposed to the virus will trigger a widespread immune response and the development of antibodies and T-cells to a point where the virus no longer spreads. Though if our immune responses fail over time and reinfection can occur, herd immunity would fail along with them.

Our understanding of SARS-CoV-2 reinfection will also have an impact on our vaccine strategy. Suppose a double dose of a Covid vaccine only protects for months instead of years. In that case, we will need to rethink the cost of manufacturing, distribution, vaccine acceptance for multiple doses, frequency of immunization, etc. Studies from the New England Journal of Medicine and Science have indicated that naturally produced antibodies may only last between two and five months. Vaccines may not be a silver bullet to stop the virus, but rather one of many tools to collectively take on the pandemic.

All this research is preliminary, and we won’t know the full extent of vaccine-based immunity until an approved vaccine has been in circulation for a long while. Perhaps the vaccine will be effective enough to expel the virus almost entirely. We will have to wait and see, but we need to understand reinfection before we can understand what the best strategy is to contain Covid, whether a vaccine or not. Be optimistic but cautious, for the pandemic is far from over.