Prevalence, incidence and longevity of antibodies against SARS-CoV-2 among primary healthcare providers in Belgium: a prospective cohort study with 12 months of follow-up

Abstract

Objectives: To estimate the prevalence, incidence and longevity of antibodies against SARS-CoV-2 among primary healthcare providers (PHCPs).

Design: Prospective cohort study with 12 months of follow-up.

Setting: Primary care in Belgium.

Participants: Any general practitioner (GP) working in primary care in Belgium and any other PHCP from the same GP practice who physically manages (examines, tests, treats) patients were eligible. A convenience sample of 3648 eligible PHCPs from 2001 GP practices registered for this study (3044 and 604 to start in December 2020 and January 2021, respectively). 3390 PHCPs (92,9%) participated in their first testing time point (2820 and 565, respectively) and 2557 PHCPs (70,1%) in the last testing time point (December 2021).

Interventions: Participants were asked to perform a rapid serological test targeting IgM and IgG against the receptor binding domain of SARS-CoV-2 and to complete an online questionnaire at each of maximum eight testing time points.

Primary and secondary outcome measures: The prevalence, incidence and longevity of antibodies against SARS-CoV-2 both after natural infection and after vaccination.

Results: Among all participants, 67% were women and 77% GPs. Median age was 43 years. The seroprevalence in December 2020 (before vaccination availability) was 15.1% (95% CI 13.5% to 16.6%), increased to 84.2% (95% CI 82.9% to 85.5%) in March 2021 (after vaccination availability) and reached 93.9% (95% CI 92.9% to 94.9%) in December 2021 (during booster vaccination availability and fourth (delta variant dominant) COVID-19 wave). Among not (yet) vaccinated participants the first monthly incidence of antibodies against SARS-CoV-2 was estimated to be 2.91% (95% CI 1.80% to 4.01%). The longevity of antibodies is higher in PHCPs with self-reported COVID-19 infection.

Conclusions: This study confirms that occupational health measures provided sufficient protection when managing patients. High uptake of vaccination resulted in high seroprevalence of SARS-CoV-2 antibodies in PHCPs in Belgium. Longevity of antibodies was supported by booster vaccination and virus circulation.

Trial registration number: NCT04779424.

Keywords: COVID-19; Epidemiology; GENERAL MEDICINE (see Internal Medicine); PRIMARY CARE.

Figure 1

Prevalence of antibodies against SARS-CoV-2 among primary healthcare providers in Belgium from December 2020 to December 2021. ¹The eight testing time points have the following start and end dates: T1: 24 December 2021–8 January 2021, T2: 25 January 2021–31January 2021, T3: 22 February 2021–28 February 2021 2021, T4: 22 March 2021–31 March 2021, T5: 19 April 2021–28 April 2021, T6: 14 June 2021–27 June 2021, T7, 13 September 2021–26 September 2021, T8: 13December 2021–26 December 2021. For the proportion of primary healthcare providers vaccinated at each testing time point, see online supplemental table S4. The green line marks the prevalence of antibodies against SARS-CoV-2 (seroprevalence). The grey line mark the 95% CI. The blue lines mark the start of primary and booster vaccination campaign for PHCPs. The grey boxes mark the third (15 February 2021–27 June 2021) and fourth COVID-19 wave (4 October 2021–27 December 2021).

Figure 2

Kaplan-Meier plot¹ of incidence of antibodies against SARS-CoV-2 among primary healthcare providers in Belgium not yet vaccinated after self-reported COVID-19 infection. ¹Interval censoring is taken into account by assuming that the actual event occurred somewhere between the testing time point of the event and the testing time point before.

Figure 3

Incidence of antibodies against SARS-CoV-2 among primary healthcare providers in Belgium after vaccination according to self-reported history of COVID-19 infection.

Figure 4

Kaplan-Meier plot¹ of longevity of antibodies against SARS-CoV-2 among PHCPs in Belgium after self-reported history of COVID-19 infection. ¹Interval censoring is taken into account by assuming that the actual event occurred somewhere between the testing time point of the event and the testing time point before.

Figure 5

Kaplan-Meier plots of longevity of antibodies against SARS-CoV-2 among primary healthcare providers in Belgium after full primary vaccination according to self-reported history of COVID-19 infection accounting for censoring as of the booster vaccination. ¹Assuming that the actual event occurred somewhere between the testing time point of the event and the testing time point before; ²Assuming that the actual event occurred exactly between the testing time point of the event and the testing time point before.