Scientists from the Danish Serum Institute analyzed nationwide health data to understand how vaccination changes the risk of re contracting COVID-19 after infection. The study used records from all residents of Denmark who tested positive for the virus between January 2020 and January 2022, totaling data from more than 200,000 individuals who had confirmed SARS CoV 2 infection. Its findings were published in PLOS Medicine and contribute to the growing picture of how vaccines work alongside natural immunity.
The results show that people who previously contracted the virus and then received vaccines have a noticeably lower chance of reinfection. During the Alpha variant period, prior infection plus vaccination reduced reinfection risk by about 71 percent. When the Delta variant circulated, protection rose to around 94 percent, and during the Omicron period the reduction stayed substantial at about 60 percent. Importantly, protection remained strong for as long as nine months after vaccination, signaling durable benefits for those with prior infection who decide to vaccinate. These figures help explain why vaccination remains a prudent choice even for those who have already fought off the virus once, and they align with independent analyses from other regions that show similar patterns of boosted protection after vaccination.
A related study conducted in Qatar found that two doses of an mRNA vaccine cut the risk of reinfection by roughly half, around 55 percent. While this Canadian and American context reinforces that vaccines provide meaningful additional protection beyond natural immunity, some limits were noted. The Danish analysis could not determine whether vaccination lowers the risk of severe outcomes such as hospitalization or death within the study period due to data duration. Still, the evidence clearly demonstrates that vaccination adds a protective layer against re exposure to SARS CoV 2 and supports vaccination campaigns as a public health measure even for those who have already built some immunity through prior infection. The takeaway for health planners in the United States and Canada is that combining natural immunity with vaccination offers the strongest shield against future infections and helps reduce onward transmission in communities over time.