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On July 31, 2021, a local cluster of COVID-19 cases was identified through PCR screening in a hospital. The virus was isolated and sequenced, showing that the cluster was caused by the Delta variant. The epidemic was managed in accordance with the Protocol for Prevention and Control of COVID-19; the last case occurred on August 24. The epidemic consisted of 167 infections, 166 were symptomatic and 1 was asymptomatic; 139 cases were in Zhengzhou. A total of 14 infections were among children below 18 years of age, and 8 were among individuals vaccinated with a vaccine other than BBIBP-CorV or CoronaVac.
For VE evaluation, 1,462 close-contacts were identified including 107 cases that were ultimately diagnosed with PCR-confirmed SARS-CoV-2 infection; 473 close contacts were unvaccinated, and 288, 455, and 246 were vaccinated with BBIBP-CorV, CoronaVac, or both vaccines (one dose of each, in either order), respectively. Table 1 shows the breakdown of the close contacts by vaccination status (unvaccinated, full, or partial vaccination), vaccine brand, age group, presence of comorbidities, and infection status with clinical outcome. Among the close contacts, 42 were HIV-positive (10 individuals) or had tuberculosis (32 individuals); 33 of these 42 close contacts had not been vaccinated, 5 had been partially vaccinated, and 4 completed 2-dose vaccination series; 35 developed infection (11 mild, 17 moderate, 6 severe, and 1 critically severe).
Type of study population Characteristics No. of
unvaccinated (%)Vaccinated with
BBIBP-CorV (%)Vaccinated with
CoronaVac (%)Vaccinated with BBIBP-CorV
and CoronaVac (%)Total (%) Partially Completely primary Partially Completely primary Partially Completely primary Partially Completely primary Close
contactsGender Male 232 (49.05) 51 (66.23) 97 (45.97) 59 (49.58) 135 (40.18) 5 (55.56) 122 (51.48) 115 (56.10) 354 (45.15) Female 241 (50.95) 26 (33.77) 114 (54.03) 60 (50.42) 201 (59.82) 4 (44.44) 115 (48.52) 90 (43.90) 430 (54.85) Age group (years) 18–59 408 (86.26) 69 (89.61) 199 (94.31) 106 (89.08) 329 (97.92) 9 (100) 212 (89.45) 184 (89.76) 740 (94.39) ≥60 65 (13.74) 8 (10.39) 12 (5.69) 13 (10.92) 7 (2.08) 0 25 (10.55) 21 (10.24) 44 (5.61) Comorbidities None 360 (76.11) 72 (93.51) 191 (90.52) 105 (88.24) 309 (91.96) 9 (100) 213 (89.87) 186 (90.73) 713 (90.94) Yes 113 (23.89) 5 (6.49) 20 (9.48) 14 (11.76) 27 (8.04) 0 24 (10.13) 19 (9.27) 71 (9.06) Total 473 77 211 119 336 9 237 205 784 People with infection Clinical outcome Asymptomatic 0 0 0 0 1 (5.55) 0 0 0 1 (2.86) Mild 13 (21.31) 1 (25.00) 3 (33.33) 3 (42.86) 10 (55.56) 0 2 (25.00) 4 (36.36) 15 (42.86) Moderate 33 (54.10) 2 (50.00) 6 (66.67) 3 (42.86) 7 (38.89) 0 4 (50.00) 5 (45.46) 17 (48.57) Severe 9 (14.75) 1 (25.00) 0 1 (14.28) 0 0 2 (25.00) 2 (18.18) 2 (5.71) Critical severe 6 (9.84) 0 0 0 0 0 0 0 0 Total 61 4 9 7 18 0 8 11 35 Table 1. Characteristics of the study population.
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Table 2 shows univariate (unadjusted) analyses of VE by vaccination status, age group, and brand of vaccine against three clinical outcomes — symptomatic COVID-19, COVID-19 pneumonia, and severe COVID-19. For adjusted vaccine effectiveness, multivariable regression analyses controlled for gender, presence of underlying conditions (comorbidities), and age group (18–59 and 60+ years).
Outcome Vaccination
history18 to 59 years old ≥60 years old Total (≥18 years old) Percentage/%
(case/total
infection)Unadjusted VE/%
(95% CI)Percentage/%
(case/total
infection)Unadjusted VE/%
(95% CI)Percentage/%
(case/total
infection)Unadjusted VE/%
(95% CI)Symptomatic
diseaseUnvaccinated 10.29 (42/408) Ref 29.23 (19/65) Ref 12.90 (61/473) Ref Partially 4.89 (9/184) 52.48 (4.45 to 76.37) 9.52 (2/21) 67.42 (−28.41 to 91.73) 5.37 (11/205) 58.39 (22.60 to 77.63) Completely primary 4.32 (32/740) 57.99 (34.53 to 73.05) 4.55 (2/44) 84.45 (36.57 to 89.63) 4.34 (34/784) 66.37 (49.65 to 77.54) BBIBP-CorV 4.52 (9/199) 56.07 (11.55 to 78.18) 0 (0/12) — 4.27 (9/211) 66.93 (34.66 to 83.26) CoronaVac 5.17 (18/329) 49.80 (13.49 to 70.88) 0 (0/7) — 5.06 (17/336) 60.77 (34.07 to 76.65) Combined 2.83 (6/212) 72.51 (36.37 to 88.12) 8.00 (2/25) 72.63 (−9.01 to 93.13) 3.38 (8/237) 73.83 (46.21 to 87.26) Pneumonia Unvaccinated 7.84 (32/408) Ref 24.62 (16/65) Ref 10.15 (48/473) Ref Partially 3.26 (6/184) 58.42 (47.91 to 83.53) 4.76 (1/21) 80.65 (−37.27 to 97.27) 3.41 (7/205) 66.35 (26.90 to 84.51) Completely primary 2.30 (17/740) 70.71 (47.91 to 83.53) 4.55 (2/44) 81.53 (23.66 to 95.53) 2.42 (19/784) 76.12 (59.88 to 85.79) BBIBP-CorV 3.02 (6/199) 61.56 (9.58 to 83.66) 0 (0/12) — 2.84 (6/211) 71.98 (35.54 to 87.82) CoronaVac 5.77 (17/329) 72.87 (39.33 to 87.87) 0 (0/7) — 2.08 (7/336) 79.47 (55.19 to 90.59) Combined 1.89 (4/212) 75.94 (32.88 to 91.38) 8.0 (2/25) 67.50 (−31.23 to 91.95) 2.53 (6/237) 75.05 (42.55 to 89.17) Severe case Unvaccinated 1.47 (6/408) Ref 13.85 (9/65) Ref 3.17 (15/473) Ref Partially 0.54 (1/184) 63.04 (−204.78 to 95.52) 4.76 (1/21) 65.61 (−155.79 to 95.38) 0.98 (2/205) 69.24 (−33.30 to 92.90) Completely primary 0 (0/740) — 4.55 (2/44) 67.17 (−44.74 to 92.55) 0.26 (2/784) 91.96 (64.98 to 98.15) BBIBP-CorV 0 (0/199) — 0 (0/12) — 0 (0/211) — CoronaVac 0 (0/329) — 0 (0/7) — 0 (0/336) — Combined 0 (0/212) — 8.0 (2/25) 42.22 (−149.05 to 86.60) 0.84 (2/237) 73.39 (−15.41 to 93.86) Note: “—” Means no data were available.
Abbreviations: CI=Confidence interval; VE=vaccine effectiveness.Table 2. Inactivated vaccine effectiveness against clinical outcomes by age group.
Table 3 shows adjusted and unadjusted VE by vaccine brand and vaccination status against the 3 clinical outcomes. Completed primary vaccination VEs for adults 18 years and over were 50.54% against symptomatic COVID-19, 61.4% against COVID-19 pneumonia, and 82.41% against severe COVID-19. Partial vaccination had non-statistically-significant VEs against the clinical outcomes.
Outcome Vaccination
historyCompletely primary vaccination Partially vaccination Percentage/%
(case/total
infection)Unadjusted VE/%
(95% CI)Adjusted VE/%
(95% CI)Percentage/%
(case/total
infection)Unadjusted VE/%
(95% CI)Adjusted VE/%
(95% CI)Symptomatic
diseaseUnvaccinated 12.90 (61/473) Ref Ref 12.90 (61/473) Ref Ref Vaccinated 4.34 (34/784) 66.37 (49.65 to 77.54) 50.54 (27.59 to 66.21) 5.37 (11/205) 58.39 (22.60 to 77.63) 33.76 (−17.53 to 62.67) BBIBP-CorV 4.27 (9/211) 66.93 (34.66 to 83.26) 50.56 (3.79 to 74.59) 5.19 (4/77) 59.72 (−7.59 to 84.92) 24.72 (−98.83 to 71.49) CoronaVac 5.06 (17/336) 60.77 (34.07 to 76.65) 39.12 (−0.91 to 63.27) 5.88 (7/119) 54.39 (2.87 to 78.58) 29.95 (−44.33 to 66.00) Combined 3.38 (8/237) 73.83 (46.21 to 87.26) 59.94 (19.09 to 80.17) 0 (0/9) — — Pneumonia Unvaccinated 10.15 (48/473) Ref Ref 10.15 (48/473) Ref Ref Vaccinated 2.42 (19/784) 76.12 (59.88 to 85.79) 61.40 (36.05 to 76.70) 3.41 (7/205) 66.35(26.90 to 84.51) 45.92 (−15.82 to 74.75) BBIBP-CorV 2.84 (6/211) 71.98 (35.54 to 87.82) 54.71 (−3.42 to 80.16) 3.90 (3/77) 61.61(−20.2 to 87.74) 16.33 (−164.3 to 73.52) CoronaVac 2.08 (7/336) 79.47 (55.19 to 90.59) 64.90 (22.88 to 84.02) 3.36 (4/119) 66.88(9.96 to 87.81) 52.65 (−25.19 to 82.09) Combined 2.53 (6/237) 75.05 (42.55 to 89.17) 62.15 (13.95 to 83.35) 0 (0/9) — — Severe case Unvaccinated 3.17 (15/473) Ref Ref 3.17 (15/473) Ref Ref Vaccinated 0.26 (2/784) 91.96 (64.98 to 98.15) 82.41 (21.03 to 96.08) 0.98 (2/205) 69.24 (−33.30 to 92.90) 44.48 (−162.62 to 88.26) BBIBP-CorV 0 (0/211) — — 1.30 (1/77) 59.05 (−205.6 to 94.51) 7.49 (−655.64 to 88.68) CoronaVac 0 (0/336) — — 0.84 (1/119) 73.5 (−98.62 to 96.46) 59.75 (−209.91 to 94.77) Combined 0.84 (2/237) 73.39 (−15.41 to 93.86) 61.35 (−71.62 to 91.30) 0 (0/9) — — Note: “—” Means no data were available.
Abbreviations: CI=Confidence interval; VE=vaccine effectiveness.Table 3. Vaccine effectiveness by brand of COVID-19 vaccine.
Table 4 shows adjusted and unadjusted VE by time between vaccination and exposure to SARS-CoV-2. Of the 784 people who completed primary vaccination, the average time from completing full vaccination (plus 14 days) to becoming a close contact was 76 days (range, 14–193 days); 572 (72.96%) completed primary vaccination three months or less before becoming a close contact, and 212 people (27.04%) had intervals 4 to 6 months following full vaccination. The adjusted VE of full vaccination against symptomatic COVID-19 was 52.32% for ≤3-month intervals and 49.95% for 4–6-month intervals (both statistically significant); against COVID-19 pneumonia, VEs were 67.08% for 4–6-month intervals; and against severe COVID-19, VEs were 80.64% for ≤3-month intervals. There were no severe cases for longer intervals.
Outcome Vaccination
historyDuration ≤ 3 months Duration 4−6 months Percentage/% (case/total
infection)Unadjusted VE/%
(95% CI)Adjusted VE/%
(95% CI)Percentage/% (case/total
infection)Unadjusted VE/%
(95% CI)Adjusted VE/%
(95% CI)Symptomatic
diseaseUnvaccinated 12.90 (61/473) Ref Ref 12.90 (61/473) Ref Ref Completely primary 4.34 (34/784) 66.11 (46.89 to 78.37) 52.32 (25.73 to 69.39) 5.37 (11/205) 67.08 (34.97 to 83.34) 49.95 (1.20 to 74.64) BBIBP-CorV 5.48 (8/146) 57.51 (13.30 to 79.18) 39.37 (−20.41 to 69.46) 1.54 (1/65) 88.07 (15.41 to 98.32) 82.00 (−25.73 to 97.42) CoronaVac 4.64 (9/194) 64.03 (29.02 to 81.77) 45.50 (−5.98 to 71.97) 5.63 (8/142) 56.32 (10.90 to 78.58) 29.83 (−41.09 to 65.11) Combined 3.45 (8/232) 73.26 (45.06 to 86.99) 59.50 (18.31 to 79.92) 0 (0/5) — — Pneumonia Unvaccinated 10.15 (48/473) Ref Ref 10.15 (48/473) Ref Ref Completely primary 2.42 (19/784) 74.16 (54.45 to 85.34) 60.31 (31.31 to 77.07) 3.41 (7/205) 81.41 (49.10 to 93.21) 67.08 (9.33 to 88.05) BBIBP-CorV 4.11 (6/146) 59.51 (7.30 to 82.31) 39.63 (−35.46 to 73.10) 0 (0/65) — — CoronaVac 1.55 (3/194) 84.76 (51.66 to 95.20) 73.85 (17.91 to 91.67) 2.82 (4/142) 72.24 (24.35 to 89.81) 47.41 (−44.35 to 80.84) Combined 2.59 (6/232) 74.52 (41.32 to 88.93) 61.79 (13.21 to 83.18) 0 (0/5) — — Severe case Unvaccinated 3.17 (15/473) Ref Ref 3.17 (15/473) Ref Ref Completely primary 0.26 (2/784) 88.97 (52.03 to 97.47) 80.64 (13.16 to 95.68) 0.98 (2/205) — — BBIBP-CorV 0 (0/146) — — 0 (0/65) — — CoronaVac 0 (0/194) — — 0 (0/142) — — Combined 0.86 (2/232) 72.81 (−17.88 to 93.73) 61.11 (−72.65 to 91.24) 0 (0/5) — — Note: “—” Means no data were available.
Abbreviations: CI=Confidence interval; VE=vaccine effectiveness.Table 4. Vaccine effectiveness by time since vaccination.
Table 5 shows severity of COVID-19 by vaccination status. The 145 infected individuals aged ≥18 years were categorized as having mild (asymptomatic and mild cases together) or pneumonia/severe (moderate, severe, and critically severe combined together) COVID-19. Compared with the unvaccinated, completed primary vaccination reduced the risk of pneumonia/severe COVID-19 by 74% [95% confidence interval (CI): 43% to 88%]. By age grouping, pneumonia/severe risk was reduced by 69% (95% CI: 27% to 87%) among 18–59-year-olds and by 77% (95% CI: –281% to 98%, not statistically significant) among people over 60 years old.
Age groups Vaccination
historyMild cases
(asymptomatic+mild)Pneumonia
(moderate+severe)OR 95% CI n % n % 18–59
years oldUnvaccinated 13 30.95 40 55.56 Ref Ref Partially 6 14.29 10 13.89 0.54 0.16 to 1.78 Completely primary 23 54.76 22 30.56 0.31 0.13 to 0.73 Sub-total 42 100 72 100 ≥60
years oldUnvaccinated 3 50.00 21 84.00 Ref Ref Partially 1 16.67 1 4.00 0.16 0.002 to 14.92 Completely primary 2 33.33 3 12.00 0.23 0.017 to 3.81 Sub-total 6 100 25 100 Total Unvaccinated 16 33.33 61 62.89 Ref Ref Partially 7 14.58 11 11.34 0.41 0.14 to 1.23 Completely primary 25 52.08 25 25.77 0.26 0.12 to 0.57 Sub-total 48 100 97 100 Abbreviations: OR=odds ratio; CI=confidential interval. Table 5. Severity of illness by vaccination status.
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Outbreak, Cases, and Subjects
Vaccine Effectiveness and Severity
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