On 24 November 2021, the World Health Organization (WHO) received its first report of the Obikerjong mutant strain from South Africa and listed it as a Variant of Concern (VOC) 2 days later. In fact, the Omicron subtype BA.2 quickly replaced BA.1 as the dominant epidemic strain. The Omicron outbreak that Hong Kong, China, is experiencing belongs to this subtype.
On 7 April 2022, Medrxiv published a research paper online from the World Health Organization Collaborating Centre for Epidemiology and Control of Infectious Diseases, School of Public Health, Li Ka Shing School of Public Health, The University of Hong Kong: Epidemiology of infections with SARS-CoV-2 Omicron BA.2 variant in Hong Kong. In this article, the researchers compared the incubation period, infection time and transmission rate of the Omicron BA.2 strain that caused the Outbreak in Hong Kong with other SARS-CoV-2 strains, and described in detail the epidemiological characteristics of the outbreak in Hong Kong.
From 2020 to 2021, Hong Kong continued to have four small outbreaks of COVID-19, with a cumulative total of 12,631 confirmed cases (1.6 per 1,000 population) and a cumulative death of 213 cases. But in early 2022, the SARS-CoV-2 Omicron BA.2 variant led to more than 1.1 million confirmed cases in Hong Kong over a three-month period, 100 times more than the number of confirmed cases in all four outbreaks in the previous two years.
Over the more than a month of the outbreak, the number of daily cases detected by RTPCR and rapid antigen testing increased exponentially, doubling in 3.4 days and peaking on 4 March 2022.
Omicron BA.2 has a strong propagation ability. On 22 January 2022, 136 cases of Omicron infection with COVID-19 were confirmed in Hong Kong, of which 72% (98/136) had a clear chain of transmission. Of the 98 confirmed cases, 34% (33) were the Omicron BA.2 variant, 55% (54) were female, and 73% had completed the primary series of vaccinations. Of these, 80 (82%) had clear symptoms at the time of diagnosis.
Next, the researchers estimated the incubation period using known exposure information and onset information from 80 patients (57 BA.1 and 23 BA.2). The average incubation period of Omicron BA.1 was 4.58 (±1.72) days and the average incubation period of Omicron BA.2 was 4.42 (±1.42) days. Symptoms developed within 7.7 days after infection with Omicron BA.1 in 95% of patients, compared with 95% of patients infected with Omicron BA.2 within 7.0 days.
Finally, the researchers used data from 43 pairs of infected people to make interval estimates of the intervals at which the Omicron variant was infected. The average interval time for BA.1 (n=30) was 3.30 (±1.95) days, and the median interval was 3.17 days. The average interval for BA.2 (n=13) is 2.72 (±1.51) days, and the median interval is 2.52 days.
In addition, the researchers also made age-stratified estimates of the case fatality rate in waves 1-4 and 5 in Hong Kong based on vaccination status. The results showed that the age-specific risk of death (CFR) in unvaccinated cases in wave 5 was similar to that in waves 1 to 4. Patients aged 80 years and older were observed to have the highest risk of death in waves 1 to 4 (CFR=24.9%), and unvaccinated patients of the same age in wave 5 showed a similar risk of death (CFR=21.7%). However, the risk of death in the unvaccinated ≥80 years of age (CFR=24.9%) in wave 5 outbreaks is about twice that of the same age group that has completed primary vaccination (CFR=11.1%), and confirmed cases aged 64-79 years have similar epidemiological characteristics.
Overall, this study shows that the Omicron mutant strain, particularly the BA.2 strain of the current Hong Kong pandemic, shows very high potential for transmission. Cases with full vaccination have a much lower risk of death compared to cases that are not vaccinated or incompletely vaccinated. The findings highlight the importance of achieving high coverage vaccination, especially in the elderly, where COVID-19 vaccination significantly reduces the risk of death.
Source: Frontiers in Life Sciences
Original title: Hong Kong's Revelation: 100-fold increase in confirmed cases, comparison of epidemiological characteristics of the Omicron subtype BA.2 variant and other NEW CORONAVIRUS variants!