RESULTS:
We enrolled 1,582 HCWs, which comprised 38.5% of eligible HCWs in
participating hospitals. Three participants were excluded because no
follow-up data were obtained after enrolment. Supplementary Figure 1
shows the flow of participants through study.
Of the 1582 HCWs enrolled, 1473 (93%) were female, the median age was
49 (IQR: 39-57); 408 (26%) were physicians, 591 (37%) were nurses, and
583 (37%) were other HCWs (Table 1). In all, 644 (41%) participants
had at least one comorbidity, and 46 (3%) and 14 (1%) participants
said they smoked currently or previously, respectively. A total of 458
(29%) HCWs had received the influenza vaccine in the 2019-2020
influenza season. Overall, 582 HCWs (37%) were from central hospitals
in Baku, while 1000 (63%) HCWs were from peripheral Baku hospitals.
At enrolment, 1040 (66%) participants had received primary series
CoronaVac, 121 (8%) participants had received only one dose of
CoronaVac, and 421 (27%) were unvaccinated (Table 1). No participants
had received other COVID-19 vaccines at enrolment. Among those
vaccinated with the primary series before enrolment, the median time
since receipt of the second dose was 99 days (IQR 74-112). Overall, 248
HCWs (16%) had a PCR-positive COVID-19 infection documented prior to
enrolment. Compared to unvaccinated participants, participants who had
received primary series vaccine before enrolment worked more commonly in
central hospitals (39% vs 29%), had more chronic condition (38% vs
48%) and had less PCR-confirmed infections prior to enrolment (6% vs
40%). Differences in participants by vaccination status at enrolment
have been previously reported.
In total, 963/1040 (93%) participants who had received primary series
vaccine were seropositive at enrolment for either anti-nucleocapsid or
the anti-spike protein antibodies; 903 (90%) were seropositive for
anti-spike antibodies, while 827 (83%) were seropositive for
anti-nucleocapsid antibodies. Among unvaccinated participants, 363/421
(86%) were seropositive to at least one of the two antibodies (Table
1). Of the 197 unvaccinated participants who did not report a previous
PCR infection at enrolment, 136 (69%) were seropositive at enrolment by
one of the two assays.
During the follow-up period, 10 (<1%) participants received
primary series vaccination with vaccines other than CoronaVac (9
received Pfizer, and 1 received Sputnik) and were excluded from the
analysis. At their exit from analysis, 1485 (95%) participants had
received primary series CoronaVac, 41 (3%) had received one dose of
CoronaVac, and 43 (3%) remained unvaccinated. Changes in the
vaccination status of the enrolled population over the course of the
study are illustrated in Figure 1. At study exit, participants who had
received the primary series were similar to unvaccinated participants in
terms of median age, sex, smoking status and self-assessed health
status; however, compared to participants who had received primary
series vaccine, unvaccinated participants were more commonly medical
doctors (47% vs 25%) and less unvaccinated participants received
influenza vaccine during the previous season (70% vs. 81%)
(Supplementary Table 1).
Unvaccinated HCWs contributed a total of 24,745 person-days to the
30-week study period, while participants vaccinated with the primary
series of CoronaVac contributed 172,457 person-days. During this time
period, 380 participants were symptomatic, of whom 201 (53%) provided a
respiratory sample that was tested for SARS-CoV-2 by PCR. Overall, 72
participants (5%) had symptomatic PCR-confirmed COVID-19, including 64
vaccinated participants and 8 unvaccinated participants. Cases peaked in
August, but incidence remained high through the end of the analysis
period; the trajectory of cases in the study mostly mirrored the
national trends of COVID-19 incidence during the summer of 2021 in
Azerbaijan (Figure 2). Of the 39 samples for which WGS data were
available, 36/39 (92%) were delta variant (Supplementary Figure 2).
During the course of the analysis period, in the 30 days following their
positive test, 44 participants with PCR-confirmed COVID-19 illness
sought medical care (4 unvaccinated and 40 fully vaccinated), and 29
participants went to an emergency room (3 unvaccinated and 26 fully
vaccinated). Three participants with PCR-confirmed COVID-19 illness were
hospitalized (1 unvaccinated and 2 fully vaccinated). No deaths occurred
among participants with PCR-confirmed COVID -19 illness. VE could not be
calculated against these more severe outcomes because of the small
number of events among unvaccinated participants.
For the overall cohort, the adjusted two-dose VE was 29% (95% CI -51%
to 67%) (Table 2, Figure 3a). For the delta-predominant period,
adjusted two-dose VE was 19% (95% CI -81% to 64%). VE was adjusted
for previous infection only; no other potential confounders changed the
VE estimates in more than 5%. For the overall cohort analysis,
vaccinated participants had received their second dose a median of 90
days (IQR: 75-112) prior to the beginning of the study period, while
during the period of delta circulation, vaccinated participants had
received their second dose a median of 108 days (IQR: 106-132) prior to
the analysis period.
When we excluded participants who had PCR-confirmed SARS-CoV-2 infection
prior to enrolment, unadjusted VE was 37 (95% CI -33 to 70), and
unadjusted VE during the delta-predominant period was 29 (95% CI -57 to
68).
For the overall cohort, adjusted VE was 39% (95% CI -40% to 73%) for
participants who had received their second CoronaVac vaccine within
14-149 days, and 19% (95%CI -81.5 to 63.4) for participants who had
received their second CoronaVac vaccine ≥150 days prior (Table 3 and
Figure 3b).
In sensitivity analyses, when we decreased the period after infection
that participants were considered not at risk from 90 to 60 days,
results were very similar (<1% difference) to the 90-day
analysis (Table 4). When we considered a participant to be fully
vaccinated at 7 days instead of 14 days, VE differed by < 2%
from the results of the primary analysis.