- Epidemic statues of influenza A and B virus
During the observation period, 10102 pharyngeal swabs were collected, including 5450 (53.9%) males and 4652 (46.1%) females, with a median age of 6 years (2 months-14 years). The distributions of daily sampling times were showed in Figure 1. Among the samples, 2899 (28.7%) were positive for Flu A, 1634 (30.0%) were male and 1265 (27.2%) were female. There were 617 positive cases of Flu B (6.1%), 336 (6.2%) males and 281 (6.0%) females.
Taking the data of influenza-like symptoms on November 28 as the baseline, there were 47 phryngeal swabs on that day, 3 (6.38%) and 3 (6.38%) were positive for Flu A and Flu B respectively. The first peak of Flu A occurred on December 2, 2019. Outpatient cases for sampling increased by 20% (1.43 folds) compared with the baseline, and the detection rate was 25.37% (3.98 folds). On December 6, 2019, it reached the second peak, with 126 sampling times (2.64 folds), 40 positive people (13.33 folds), and the detection rate of 32.26% (5.06 folds). The highest positive rate of Flu A occurred on December 15, 2020, reaching to 40.12% (6.29 folds). 132 children with Flu A were positive, 44 folds as many as the number of the initial stage. The number of pharygeal swabs was the most on December 22, 2019, with 412 samples per day, and the detection rate was 28.13%. After January 5, 2020, the detection rate of Flu A dropped to below 25%. On January 10, 2020, the number of samples detected was reduced to less than 100. By the end of the observation period, on January 20, 2020, the number of sampling visits and the detection rate of Flu A were reduced to 46 and 4.35% respectively, returning to the level of the initial stage of the epidemic.
During the same time period, the detection rate of Flu B remained a stable proportion, with an median range of daily detection rate of 6.06% (3.29% - 16.67%), while the detection rate was negatively correlated with Flu A (CI 95%, r = -0.3908, P = 0.0026). In the early and late stage of this Flu A epidemic, the detection rate of Flu B is significantly increase, which can reach to more than 10% of the detected population. Therefore, during the period of intensive incidence of Flu A, there was still a low intensity epidemic of Flu B.
- Detection rate of influenza A and B virus in different ages
According to the age classification, the detection rate of Flu A was lowest in the group less than 1 year old (18.40%), 25% in the group of 3 years old and above, and 32.07% in the group of 12 years old. According to gender classification, the detection rate in boys younger than 1 year old was 19.11%, 2 years old group was 24.30%, 3 years old and above group was more than 25%; The distribution of girls was similar to that of boys, but the detection rate of 2 years old group was less than 20%, and that of 3 years old and above group was more than 25%.These results showed that children over 3 years old were more susceptible to Flu A (Table 1).
There was a low level of infection of Flu B during the high prevalence period of Flu A. the detection rate of each age group fluctuated between 3% and 10% (Table 2). But only 1.9% of the children were positive in the 13 years old group and 0 in the 14 year old group. The epidemic intensity of influenza B was negatively correlated with influenza A (P < 0.01). In addition, both Flu A and B antigens (48, 0.48%) were detected in the same pharygeal swab, indicating that there may be presence of mixed infection. Two children were detected with H7 antigen in December 2019.
- Time of influenza A and B virus antigens turning negative
The time from positive to negative was recorded in 1191 patients. The median time of Flu A antigen turning negative was 6 days (1-30 days). 73.6% of pharygeal swabs turned negative within 7 days, and 3.7% of pharygeal swabs remained positive for more than 14 days. Among them, 4 children aged 2-4 years old with upper respiratory tract infection were persistent positive over 30 days after symptoms disappeared. 192 cases Flu B completed twice tests. The median time of antigen turning negative was 6 days (2-19 days). These results showed that the turning negative time of Flu A and B virus in pharygeal swabs of most children was less than 7 days, but the survival time of respiratory influenza virus in a few children can last for more than 1 month.
- Alternation of influenza A epidemic and COVID-19 outbreak
The prevalence of Flu A in children in 2019 is different from that in the past, with highly overlaps the early spread of SARS-CoV-2 in terms of epidemic time. Since COVID-19 is a sudden outbreak, we conducted a retrospective study on the pharygeal swab samples of children kept in our laboratory at the same time (Table 3).There were 19 boys and 16 girls, the median age was 7.7 years (2 months-14y). Fever or cough was the chief complaint in most cases, accounting for 91.4% (32 / 35) of fever and 31.4% (11 / 35) of cough. Among the 35 pharygeal swabs, 11 were positive for Flu A (31.42%). No Flu B was detected. All pharygeal swabs of children were negative for SARS-CoV-2 nucleic acid screening. According to the date of first confirmed COVID-19, the cases were divided into two groups. There was no significant difference between the two groups in clinical characteristics, absolute values, percentages of peripheral blood leukocytes and lymphocytes. The results showed that there was no co infection of SARS-CoV-2 and influenza A in children with respiratory symptoms in Wuhan at the beginning of COVID-19 outbreak.