Researchers from the University of Toronto have found that people who caught COVID-19 during the pandemic’s first wave in 2020 appeared to have a noticeably lower chance of catching the virus again in the early Omicron period. Specifically, those individuals were about 40 percent less likely to report a reinfection in the first half year of Omicron’s spread, compared with those who had never been sick in the initial wave. The findings were prepared for presentation at ECCMID 2023, the European Congress of Clinical Microbiology and Infectious Diseases.
The analysis drew on data from 618 patients who recovered during the pandemic’s first wave, spanning March through September 2020, and 168 adults who did not experience a COVID-19 infection during that same window. The two groups shared a similar average age, approximately 56 or 57 years old, which helped ensure a balanced comparison of outcomes across age ranges.
In comparing adults who had recovered from the first wave with those who were never infected, researchers observed that the former group faced about a 40 percent lower risk of catching the Omicron variant during its surge, roughly from December 2021 through May 2022. These results underscore the potential lasting protection linked to a prior infection, even as the virus evolved into a new dominant variant.
Age played a notable role in risk patterns. People aged 65 and older experienced a substantially reduced chance of Omicron infection, with a decrease of around 66 percent. Importantly, this lower risk did not hinge on whether they had COVID-19 during the first wave, suggesting a broad protective effect that persisted across different infection histories for older adults.
As expected, vaccination continued to contribute meaningful protection. The study found that vaccination, along with inherent immune responses, lowered the risk of infection by a range of about 24 to 65 percent during the Omicron period. Together, these factors highlight how prior infection, vaccination, and age-related immune factors interact to influence susceptibility as the virus evolves. The evidence supports a layered approach to reducing transmission and severe outcomes, even when new variants emerge and spread within diverse population groups.