Compared to adults who did not have COVID in the first wave (March to September 2020), adults infected with COVID-19 in that first wave were 40% less likely to become infected during the first six months of Omicron activity (December 2021 to May 2022), concludes a new Canadian study to be presented at this year's European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Copenhagen, Denmark (15-18 April). The study was led by Dr Allison McGeer, Sinai Health System, University of Toronto, Canada, and colleagues.
In addition, vaccination, as expected, also offered substantial protection on top of natural immunity. Age was also a factor: older adults (both 65 years and older and aged 50-64 years) were less likely to be infected than younger adults aged 18-49 years in this first six months of the Omicron wave (dominated by BA.1 and BA.2 sublineages).
Patients infected with SARS-CoV-2 between March and September of 2020 were enrolled along with a cohort who were matched on hospitalisation status, age-group, and indication for and timing of SARS-CoV-2 testing who did not have an early infection. Participants completed baseline questionnaires and were followed until May 31, 2022, to identify any respiratory illness, positive COVID test results (PCR or rapid test) and the number and timing of COVID-19 vaccine doses received.
The primary outcome was infection with SARS-CoV-2 between Dec 15, 2021, and May 31,
2022 (the BA.1/BA.2 period). The primary analysis excluded 18 people across both groups who had SARS-COV-2 infections between October 2020 (after the first wave ended) and December 15, 2021(before the Omicron wave started) (8 in the EI group, 10 in the non-EI group), so that the study could make a direct comparison for protection against Omicron between those infected or not infected in the first wave. Multivariable modelling was used to estimate the risk ratio of infection comparing those with and without early SARS-CoV-2 infections, adjusted for age, sex, immunosuppression, household income, calendar time (weeks), number of vaccine doses received, and time from most recent dose.
Data were available for 618 participants with early infection (EI) (282 inpatients and 336 outpatients) and 168 participants without (non-EI). Median age was 56 years for EI and 57 years for non-EI. 325 (53%) EI and 93 (55%) non-EI participants were female. 19 (3%) EI and 8 (5%) non-EI participants were immunocompromised. Among those with complete vaccination information, 539/572 (94%) EI and 136/146 (96%) non-EI participants had received 2 or more doses of a COVID-19 vaccine prior to the onset of the Omicron wave.
During the BA.1/BA.2 period, 84 EI and 36 non-EI participants developed SARS-CoV-2 infections, an incidence of 2.9/100 person-months in the EI cohort versus 4.8/100 person-months in the non-EI cohort. Overall, individuals infected in the first wave early in the pandemic (March to September 2020) had a 40% lower risk of SARS-CoV-2 infection during the Omicron wave than those who were not infected early in the pandemic.
Independent of the presence of early infection, older adults were less likely to be infected in the Omicron wave. Compared with adults aged 18-49 years, those aged 50-64 years were 36% less likely to be infected, and those aged 65 years and older 66% less likely.
Vaccination was also associated with an estimated 24% to 65% reduction in infection during the Omicron wave. The very small number of unvaccinated individuals made estimates of the degree of protection from different numbers of doses and at different times imprecise.
The authors conclude: "Infection with the original first-wave SARS-CoV-2 virus during March to September 2020 was associated with a 40% reduction in the risk of infection during the Omicron BA.1/BA.2 period from December 2021-May 2022. COVID-19 vaccination conferred additional protection, and our study showed younger individuals to be more at risk of infection by Omircon than older age groups."
