Baseline Anti-Spike IgG Assays and PCR Test Rates
A total of 12,541 healthcare professionals underwent measurement of baseline anti-spike antibodies; 11,364 (90.6%) were seronegative and 1177 (9.4%) were seropositive at their first anti-spike IgG assay, and seroconversion occurred in 88 workers during the study (table 1and fig. S1A in the Supplementary Appendix). Of the 1265 seropositive healthcare professionals, 864 (68%) reported having symptoms consistent with those of coronavirus disease 2019 (Covid-19), including symptoms that preceded the widespread availability of PCR testing for SARS-CoV. -2; 466 (37%) had a previous PCR-confirmed SARS-CoV-2 infection, of which 262 were symptomatic. Fewer seronegative health workers (2860 [25% of the 11,364 who were seronegative]) reported pre-baseline symptoms, and 24 (all symptomatic, 0.2%) were previously PCR-positive. The median age of seronegative and seropositive health professionals was 38 years (interquartile range, 29 to 49). Healthcare professionals were followed for a median of 200 days (interquartile range, 180 to 207) after a negative antibody test and for 139 days at risk (interquartile range, 117 to 147) after a positive antibody test.
The frequency of symptomatic PCR testing was similar in seronegative and seropositive healthcare professionals: 8.7 and 8.0 tests per patient, respectively. 10,000 risk days (rate ratio, 0.92; 95% confidence interval [CI](0.77 to 1.10). A total of 8850 health workers had at least one asymptomatic baseline screening test; seronegative health professionals participated in asymptomatic screening more often than seropositive health professionals (141 vs. 108 per 10,000 risk days, respectively; rate ratio, 0.76; 95% CI, 0.73 to 0.80).
Presence of PCR-positive results according to Baseline Anti-Spike IgG status
Positive baseline anti-spike antibody assays were associated with lower frequencies of PCR-positive tests. Of 11,364 healthcare professionals with a negative antispitus IgG assay, 223 had a positive PCR test (1.09 per 10,000 days at risk), 100 under asymptomatic screening, and 123 while symptomatic. Out of 1265 healthcare professionals with a positive anti-spike IgG assay, 2 had a positive PCR test (0.13 per 10,000 days at risk) and both workers were asymptomatic when tested. Incidence rate for positive PCR tests in seropositive workers was 0.12 (95% CI, 0.03 to 0.47; P = 0.002). The incidence of PCR-confirmed symptomatic infection in seronegative healthcare professionals was 0.60 per 10,000 days at risk, while there were no confirmed symptomatic infections in seropositive healthcare professionals. No PCR-positive results occurred in 24 seronegative, former PCR-positive health workers; seroconversion occurred in 5 of these workers during follow-up.
The incidence of polymerase chain reaction (PCR) test positive for SARS-CoV-2 infection in the period from April to November 2020 is shown per 10,000 days at risk among healthcare professionals according to their antibody status at baseline. In seronegative health professionals, 1775 PCR tests (8.7 per 10,000 risk days) were performed in symptomatic individuals and 28,878 (141 per 10,000 risk days) in asymptomatic individuals; in seropositive health professionals, 126 (8.0 per 10,000 risk days) were performed in symptomatic individuals and 1704 (108 per 10,000 risk days) in asymptomatic individuals. RR denotes rate ratios.
Occurrence varied by calendar time (figure 1), reflecting the first (March to April) and second (October and November) wave of the UK pandemic and was consistently higher among seronegative health professionals. After adjusting for age, sex, and test month (Table S1) or calendar time as a continuous variable (Fig. S2), the incidence rate for seropositive workers was 0.11 (95% CI, 0.03 to 0.44; P = 0.002). The results were similar in analyzes where follow-up of both seronegative and seropositive staff began 60 days after baseline serological analysis; with a 90-day window after positive serological analysis or PCR test; and after random removal of PCR results for seronegative health professionals to match asymptomatic test rates in seropositive health professionals (Tables S2 to S4). The occurrence of positive PCR tests was inversely associated with anti-spike antibody titers, including titers below the positive threshold (P <0.001 for trend) (Fig. S3A).
Anti-nucleocapsid IgG status
With anti-nucleocapsid IgG used as a marker of previous infection in 12,666 healthcare professionals (Fig. S1B and Table S5), 226 of 11,543 (1.10 per 10,000 days at risk) tested seronegative healthcare professionals PCR-positive compared to 2 in 1172 (0, 13 per 10,000 days at risk) antibody-positive healthcare professionals (incidence rate adjusted for calendar time, age, and sex, 0.11; 95% CI, 0.03 to 0.45; P = 0.002) (Table S6). The incidence of PCR-positive results decreased with increasing anti-nucleocapsid antibody titers (P <0.001 for trend) (Fig. S3B).
A total of 12,479 health workers had both antispike and anti-nucleocapsid baseline results (Figs. S1C and Tables S7 and S8); 218 out of 11,182 workers (1.08 per 10,000 risk days) with both immunoassays negative had subsequent PCR-positive tests compared with 1 in 1021 workers (0.07 per 10,000 risk days) with both baseline analyzes positive (incidence rate, 0 , 06; 95% CI, 0.01 to 0.46) and 2 out of 344 workers (0.49 per 10,000 risky days) with mixed antibody assay results (incidence rate, 0.42; 95% CI, 0.10 to 1 , 69).
Seropositive health workers with PCR-positive results
Three seropositive health workers subsequently had PCR-positive tests for SARS-CoV-2 infection (one with anti-spike IgG only, one with anti-nucleocapsid IgG only, and one with both antibodies). The time between initial symptoms or seropositivity and subsequent positive PCR testing ranged from 160 to 199 days. Information on the clinical history of the workers and on the results of PCR and serological tests are shown in Table 2 and Figure S4.
Only healthcare professionals with both antibodies had a history of PCR-confirmed symptomatic infection that preceded serological testing; after five negative PCR tests, this worker had a positive PCR test (low viral load: cycle number, 21 [approximate equivalent cycle threshold, 31]) on day 190 after infection while the worker was asymptomatic, with subsequent negative PCR tests 2 and 4 days later and no subsequent increase in antibody titers. If this worker’s single PCR positive result was false positive, the incidence rate of PCR positivity if anti-spike IgG seropositive would decrease to 0.05 (95% CI, 0.01 to 0.39) and if anti-spike IgG seropositive nucleocapsid IgG seropositive would decrease to 0.06 (95% CI, 0.01 to 0.40).
A fourth double-seropositive health worker had a PCR-positive test 231 days after the worker’s index symptomatic infection, but retesting the worker’s sample was negative twice, indicating a laboratory error in the original PCR result. Subsequent serological analyzes showed decreasing anti-nucleocapsid and stable anti-spike antibodies.