Table 4. SARS-CoV-2 seroconversion rates at scheduled visits 2 to 6.

	Anti-N IgG and/or anti-S IgM		Anti-N IgG**			Anti-S IgM at visits 2–3, and/or anti-N IgG at visits 2–6
	Visit 2	Visit 3	Visit 4	Visit 5	Visit 6	Overall
Population at risk*	3564	3496	3448	3169	2049	NA
Total participants who seroconverted	36	53	32	19	123	263***
Median follow-up time (months)	1.2	1.2	1.4	4.8	7.9	1.4
Total follow-up (person-months)	4,512	4,299	4,963	11,485	14,522	39,781
Seroconversion rate (per 1000 person-months) [95% CI]	8.0 [5.6; 11.1]	12.3 [9.2; 16.1]	6. 5 [4.4; 9.1]	1.7 [1.0; 2.6]	8.5 [7.0; 10.1]	6.6 [5.8; 7.5]

CI: confidence interval.

* Total number of subjects with negative serostatus at previous visit.

** At visits 4–6, seroconversion was restricted to positive anti-N IgG antibodies, which are specific for infection-induced immune response and, therefore, not subject to vaccine-induced immune response. Since COVID-19 vaccines were not yet available during visits 2–3, seroconversion was also defined as having positive anti-S IgM antibodies.

*** 255 participants seroconverted for anti-N IgG (at visits 2–6) and/or anti-S IgM (at visits 2–3), of whom eight seroconverted twice. Hence, there were 263 seroconversion events in total.