[Analysis on the epidemic characteristics and genetic characteristics of varicella in Beijing from 2019 to 2021].

The reported incidence of varicella in Beijing from 2019 to 2021 were 63.8/100 000, 32.2/100 000 and 38.6/100 000, respectively. There were two VZV epidemics in Beijing each year, one peaked in May and the other in November. However, the first VZV epidemic almost disappeared in 2020. Among the cases involved in the varicella outbreaks in school, the proportion of the students with no history of vaccine immunization, 1 dose of immunization and 2 doses of immunization were 33.12%, 44.79% and 22.08%, respectively. The major body of VZV breakthrough cases was children aged 6-14 years (523/755, 69.27%). The proportion of moderate- or severe-rash were 55.32%, 39.06%, 29.96% in the three groups of cases with no immunization history, 1 dose of immunization and 2 doses of immunization, respectively (<0.001). A total of 1 089 varicella samples were collected, and 837 (76.86%) were confirmed to be PCR-positive for VZV and were identified as VZV wild strains. 311 VZV strains were sequenced successfully, and 307 strains were clade 2 (98.72%), 1 clade 3 (0.32%) and 3 Clade 5 (0.96%). Compared with the representative strains, the nucleotide similarities of ORF22 fragments were between 99.4% and 100%, and amino acid similarities were between 99.4% and 100%.