Risk and symptoms of COVID-19 in health professionals according to baseline immune status and booster vaccination during the Delta and Omicron waves in Switzerland—A multicentre cohort study

Abstract

Background Knowledge about protection conferred by previous Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection and/or vaccination against emerging viral variants allows clinicians, epidemiologists, and health authorities to predict and reduce the future Coronavirus Disease 2019 (COVID-19) burden. We investigated the risk and symptoms of SARS-CoV-2 (re)infection and vaccine breakthrough infection during the Delta and Omicron waves, depending on baseline immune status and subsequent vaccinations. Methods and findings In this prospective, multicentre cohort performed between August 2020 and March 2022, we recruited hospital employees from ten acute/nonacute healthcare networks in Eastern/Northern Switzerland. We determined immune status in September 2021 based on serology and previous SARS-CoV-2 infections/vaccinations: Group N (no immunity); Group V (twice vaccinated, uninfected); Group I (infected, unvaccinated); Group H (hybrid: infected and ≥1 vaccination). Date and symptoms of (re)infections and subsequent (booster) vaccinations were recorded until March 2022. We compared the time to positive SARS-CoV-2 swab and number of symptoms according to immune status, viral variant (i.e., Delta-dominant before December 27, 2021; Omicron-dominant on/after this date), and subsequent vaccinations, adjusting for exposure/behavior variables. Among 2,595 participants (median follow-up 171 days), we observed 764 (29%) (re)infections, thereof 591 during the Omicron period. Compared to group N, the hazard ratio (HR) for (re)infection was 0.33 (95% confidence interval [CI] 0.22 to 0.50, p < 0.001) for V, 0.25 (95% CI 0.11 to 0.57, p = 0.001) for I, and 0.04 (95% CI 0.02 to 0.10, p < 0.001) for H in the Delta period. HRs substantially increased during the Omicron period for all groups; in multivariable analyses, only belonging to group H was associated with protection (adjusted HR [aHR] 0.52, 95% CI 0.35 to 0.77, p = 0.001); booster vaccination was associated with reduction of breakthrough infection risk in groups V (aHR 0.68, 95% CI 0.54 to 0.85, p = 0.001) and H (aHR 0.67, 95% CI 0.45 to 1.00, p = 0.048), largely observed in the early Omicron period. Group H (versus N, risk ratio (RR) 0.80, 95% CI 0.66 to 0.97, p = 0.021) and participants with booster vaccination (versus nonboosted, RR 0.79, 95% CI 0.71 to 0.88, p < 0.001) reported less symptoms during infection. Important limitations are that SARS-CoV-2 swab results were self-reported and that results on viral variants were inferred from the predominating strain circulating in the community at that time, rather than sequencing. Conclusions Our data suggest that hybrid immunity and booster vaccination are associated with a reduced risk and reduced symptom number of SARS-CoV-2 infection during Delta- and Omicron-dominant periods. For previously noninfected individuals, booster vaccination might reduce the risk of symptomatic Omicron infection, although this benefit seems to wane over time.

Efficacy of the neutralizing antibodies after the booster dose on SARS-CoV-2 Omicron variant and a two-year longitudinal antibody study on Wild Type convalescents

“…This is related to the protective effects of the booster vaccination. Our findings are in line with the results of a multicenter cohort study in Switzerland, which reported that hybrid immunity (infection plus vaccination) and receipt of booster vaccination resulted in fewer symptoms (median, 3) than unvaccinated participants (median, 4) [10] . It has been reported that lower NAb levels are associated with an increased risk of SARS-CoV-2 infection and a higher disease burden [11] .…”

Longitudinal study of humoral immunity against SARS-CoV-2 of health professionals in Brazil: the impact of booster dose and reinfection on antibody dynamics

“…Therefore, our study demonstrates that prolonged protection after COVID-19 infection, even after the booster dose, does not prevent reinfection by new variants, which contrasts with the prolonged immune response cited by other studies (66,80). Studies show that previous infection and booster dose reduce the risk of reinfection, as seen in Switzerland (81), Qatar (61), and United States of America (82). However, studies have shown a significant decrease in the effectiveness of the vaccine against the Omicron variant within a few months after administration (83).…”

Clinical symptoms of SARS‐CoV‐2 breakthrough infection during the Omicron period in relation to baseline immune status and booster vaccination—A prospective multicentre cohort of health professionals (SURPRISE study)

“…As a result, the prevalence of hybrid immunity from vaccination in combination with prior or subsequent infection in the population increases progressively 3 . Concomitantly, recent data suggest that hybrid immunity from exposures to the SARS‐CoV‐2 virus and the vaccine confers the highest level of protection from Omicron infection 1,2,5–7 …”

“…3 Concomitantly, recent data suggest that hybrid immunity from exposures to the SARS-CoV-2 virus and the vaccine confers the highest level of protection from Omicron infection. 1,2,[5][6][7] Compared with the high overall burden of disease, there is a scarcity of data assessing whether individual symptoms of breakthrough infections differ between subjects with pre-existing hybrid or vaccineinduced immunity and whether these symptoms are affected by booster vaccine doses. In this study, we aimed to assess the effects of these different types of pre-existing immunity on the frequency of clinical symptoms caused by the SARS-CoV-2 breakthrough infection during the Omicron period, with or without additional booster vaccine doses.…”