Epidemiology of Infections with SARS-CoV-2 Omicron BA.2 Variant, Hong Kong, January-March 2022

Abstract

Our analysis of data collected from multiple epidemics in Hong Kong indicated a shorter serial interval and generation time of infections with the SARS-CoV-2 Omicron variant. The age-specific case-fatality risk for Omicron BA.2.2 case-patients without complete primary vaccination was comparable to that of persons infected with ancestral strains in earlier waves.

Keywords: BA.2; COVID-19; China; Hong Kong; Omicron; SARS-CoV-2; coronavirus disease; epidemiology; fatality; respiratory infections; severe acute respiratory syndrome coronavirus 2; vaccine-preventable diseases; viruses; zoonoses.

Figure 1

Confirmed COVID-19 local cases and deaths in Hong Kong over the first 4 epidemic waves (A) and wave 5 (B, C). Blue and red bars in panels A and B indicated daily numbers of COVID-19 cases confirmed by reverse transcription PCR and daily number of reported deaths. Bars in light blue in panel B indicate daily numbers of COVID-19 cases detected by rapid antigen tests. Panel C indicates the exponential growth of the reported COVID-19 cases in wave 5 before the epidemic peaked. Six fatal imported cases were also included in panel A; no deaths were reported among persons with imported cases in wave 5.

Figure 2

Age-stratified estimates of the case-fatality risk for COVID-19 in epidemic waves 1–4 and wave 5 in Hong Kong by vaccination status. Case-patients were classified as having a complete primary series if they had received >2 doses of COVID-19 vaccines >2 weeks before symptom onset (for symptomatic case-patients) or >3 weeks before laboratory confirmation of the infection (for symptomatic case-patients with a missing onset date or asymptomatic case-patients), otherwise as having an incomplete primary series. The COVID-19 vaccines available in Hong Kong included BNT162b2 (Pfizer-BioNTech, https://www.pfizer.com) and CoronaVac (Sinovac, https://www.sinovac.com) vaccines.