Table 1.

Study Follow-Up Groups

Study Group	Description	Start of At-Risk Period	End of At-Risk Period, Study End or . . .
Unvaccinated, seronegative	Unvaccinated, consistently seronegative	The day of first negative antibody test	Positive PCR test
			First vaccination
			Positive antibody test
Unvaccinated, seropositive	Unvaccinated, and ever seropositive	>60 d after their first pre-vaccinated positive antibody testa	Positive PCR test
			First vaccination
Vaccinated once, previously seronegative	Vaccinated once, always seronegative prior to vaccination	>14 d after first vaccine dose	Positive PCR test
			Second vaccination
Vaccinated twice, previously seronegative	Vaccinated twice, always seronegative prior to vaccination	>14 d after second vaccine dose	Positive PCR test
Vaccinated, previously seropositive	Vaccinated (once or twice), and ever seropositive prior to first vaccination	>14 d after first vaccine dose	Positive PCR test

Abbreviation: PCR, polymerase chain reaction.

^aTo allow for any persistent RNA from the first infection and also requiring >60 days since the last positive PCR test. Those who were vaccinated without any prior antibody measurement were included in the previously seronegative follow-up groups.