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BackgroundThe presymptomatic transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been documented in limited clusters, and it is predicted through modelling. However, there is a lack of evidence from observations with a large sample size.MethodsWe used data from meticulous contact tracing of people exposed to cases of SARS-CoV-2 to estimate the proportion of cases that result from the presymptomatic transmission of the virus in Beijing during January 2020 and February 2020.ResultsThe results showed that presymptomatic transmission occurred in at least 15% of 100 secondary COVID-19 cases. The earliest presymptomatic contact event occurred 5 days prior to the index case’s onset of symptoms, and this occurred in two clusters.ConclusionsThe finding suggested that the contact tracing period should be earlier and highlighted the importance of preventing transmission opportunities well before the onset of symptoms.

ABSTRACTBackgroundSince 2007, trivalent inactivated influenza vaccine (TIV) has been provided free-of-charge to primary, middle school and high school students in Beijing. However, there have been few school-based studies on influenza vaccine effectiveness (VE). In this report, we estimated influenza VE against laboratory-confirmed influenza illness among school children in Beijing, China during the 2016–2017 influenza season.MethodsThe VE of 2016–2017 TIV against laboratory-confirmed influenza virus infection among school-age children was assessed through a case–control design. Conditional logistic regression was conducted on matched case–control sets to estimate VE. The effect of prior vaccination on current VE was also examined.ResultsAll 176 samples tested positive for influenza A virus with the positive rate of 55.5%. The average coverage rate of 2016–2017 TIV among students across the 37 schools was 30.6%. The fully adjusted VE of 2016–2017 TIV against laboratory-confirmed influenza was 69% (95% CI: 51 to 81): 60% (95% CI: −15 to 86) for influenza A(H1N1)pdm09 and 73% (95% CI: 52 to 84) for influenza A(H3N2). The overall VE for receipt of 2015–2016 vaccination only, 2016–2017 vaccination only, and vaccinations in both seasons was 46% (95% CI: −5 to 72), 77% (95% CI: 58 to 87), and 57% (95%CI: 17 to 78), respectively.ConclusionsOur study during school outbreaks found that VE of 2016–2017 TIV was moderate against influenza A(H3N2) as well as A(H1N1)pdm09 viruses.

ABSTRACTBackground:Little is known about the vaccine effectiveness (VE) in attenuating the influenza-associated symptoms in children during the 2014–2015 influenza season in Beijing, China, in which there was a mismatch between the vaccine and circulating strain.Methods:This study included 210 laboratory-confirmed influenza cases among children, who were enrolled in 2014, from November 1 to December 31. Standard demographic information and clinical symptoms were recorded. Influenza vaccination was confirmed via a vaccination registry. Univariate and multivariate analyses were used to estimate the odds of presenting with clinical symptoms among vaccinated and unvaccinated groups, adjusting for sex, age, area, BMI level, and chronic conditions.Results:Among the 210 laboratory-confirmed influenza cases, 170 (81.0%) presented with fever ≥38°C. The other most common symptoms were cough (78.1%), sore throat (46.7%), nasal congestion/rhinorrhea (38.6%), headache (34.8%), fatigue (24.8%) and myalgia/arthralgia (16.2%). Approximately 9.0% (19/210) exhibited nausea/vomiting, and 2.4% (5/210) exhibited diarrhea/abdominal pain. Respiratory complications occurred in 5.7% (12/210) of the confirmed influenza cases. In 210 laboratory-confirmed cases, univariate and multivariate conducted after adjusted for the aforementioned characteristics suggested that the odds of fever ≥ 38°C were significantly reduced in vaccinated children (odds ratio [OR]: 0.42, 95% CI: 0.19–0.93; P= 0.033).Conclusions:Influenza vaccination may reduce the clinical symptoms of laboratory-confirmed influenza cases potentially even in the mismatching season.

ABSTRACTBackground: Since 2007, trivalent inactivated influenza vaccine has been provided free-of-charge to older adults aged ≥60 years in Beijing, China, but the data regarding influenza vaccine effectiveness (VE) among these people are very limited so far. We sought to estimate influenza VE against medically-attended laboratory-confirmed influenza illness among older adults during the 2013–2014 season.Methods: The influenza-like illness (ILI) patients aged 60 years and older who participated in the influenza virological surveillance of Beijing during 2013–2014 influenza season were recruited in this study. A test-negative design was employed to estimate influenza VE among older adults by using logistic regression models. VE was estimated using logistic regression, adjusted for sex, age, interval (days) between illness onset and specimen collection, and week of illness onset.Results: Between 1 November, 2013 and 30 April, 2014, a total of 487 elderly ILI patients were enrolled in the study, including 133 influenza-positive cases (of whom 6.8% were vaccinated) and 354 influenza-negative controls (of whom 10.2% were vaccinated). Among 133 influenza-positive cases, 51 tested positive for A(H1N1)pdm09 virus, 22 positive for A(H3N2) virus, 52 tested positive for B/Yamagata-lineage virus, 2 positive for B/Victoria-lineage virus, 1 positive for both A(H1N1)pdm09 and A(H3N2) viruses, and 5 tested positive for viruses of unknown subtype or lineage. The adjusted overall VE was estimated as 32% (95% CI:-48-69), with 59% (95% CI: −79–90) against A(H1N1)pdm09, 22% (95% CI: −253-83) against A(H3N2) and −20% (95% CI: −239-58) against B/Yamagata-lineage viruses.Conclusions: These results suggested a modest protective effect of the 2013–2014 influenza vaccine among older adults in Beijing which was not statistically significant, with higher VE against the A(H1N1)pdm09 viruses compared to A(H3N2) and B viruses.

ABSTRACTInfluenza vaccination is the most effective way of preventing influenza infections but its coverage is extremely low in China. Poor influenza vaccine effectiveness (VE) was reported in the 2014/15 season in some Northern Hemisphere countries with a predominance of H3N2 viruses belonging to the 3C.2a clade. However, there is limited information regarding the preventive effect of influenza vaccination for the same season in China, in which H3N2 viruses belonging to the 3C.3a clade predominated. Through influenza virological surveillance in Beijing, China during the 2014/15 season, we estimated the influenza VE against medically attended influenza-like illness (ILI) associated with laboratory-confirmed influenza virus infection using a test-negative design, and the effect of prior vaccination on current vaccination was examined. In total, 9297 patients with ILI were enrolled in this study. Among them, 3434 (36.9%) tested positive for influenza viruses: 2167 (63.1%) for A(H3N2), 1261 (36.7%) for influenza B, and 3 (0.1%) for A(H1N1)pdm09. The adjusted VE was estimated as −25% (95% CI: −70%, 8%) against A(H3N2) and −8% (95% CI: −50%, 23%) against B, with an overall VE of −18% (95% CI: −49%, 6%). The overall VE estimate for patients who received 2014/15 vaccination only was −12% (−57%, 20%), while VE for patients who received both 2013/14 and 2014/15 vaccinations was −27% (−72%, 7%). There was no evidence that the influenza vaccine protected vaccinees against medically attended influenza in Beijing, China during the 2014/15 season.