The epidemiological features of SARS-CoV-2 infections in the fifth wave in Hong Kong

A recent report posted to the medRxiv* preprint server presented the epidemiology of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron BA.2 sublineage infections in Hong Kong.

Study: Epidemiology of infections with SARS-CoV-2 Omicron BA.2 variant in Hong Kong, January-March 2022. Image Credit: Yung Chi Wai Derek/Shutterstock


Following the emergence of the SARS-CoV-2 original strain in early 2020, many SARS-CoV-2 variants of concern have led to big outbreaks and significant deaths. The SARS-CoV-2 Omicron variant first detected in South Africa and Botswana in late November 2021 is the latest SARS-CoV-2 variant to spread internationally.

In the first two years of the coronavirus disease 2019 (COVID-19) pandemic, Hong Kong recorded 12,631 confirmed SARS-CoV-2 cases and 213 SARS-CoV-2-associated mortality. By December 31, 2021, 70% of Hong Kong citizens had received two doses of COVID-19 vaccination. Yet, in early 2022, Hong Kong saw a big COVID-19 wave, primarily driven by the SARS-CoV-2 Omicron BA.2.2 variant, with more than 1.1 million reported SARS-CoV-2 infections and over 7900 deaths.

About the study

In the present study, the scientists profiled the epidemiological characteristics of the SARS-CoV-2 infections in the COVID-19 pandemic's fifth wave (January to March 2022) in Hong Kong. 

The authors estimated the incubation period of 80 individuals infected with the SARS-CoV-2 Omicron variant with known symptom onset and exposure during the fifth COVID-19 wave in Hong Kong. The team assessed the serial interval of COVID-19 using details of 43 symptomatic SARS-CoV-2 infectors to infectee pairs. Further, the Omicron BA.1 variant generation time was computed utilizing datasets from 45 infector-infectee pairings with defined exposure-to infection time. In addition, the serial interval and generation time distributions of the Omicron variant were assessed using the Weibull distribution.

The team stratified COVID-19 cases as vaccinated with primary series SARS-CoV-2 vaccines if patients had received two shots of the inactivated CoronaVac or messenger ribonucleic acid (mRNA) BNT162b2 vaccine, with a most recent dose around two weeks prior. The researchers predicted that the age-specific case-fatality-risk (CFR) without completion of the main series of immunization in the fifth COVID-19 wave was similar to those verified in the SARS-CoV-2 pandemic one to four waves throughout all age groups after adjusting for unresolved outcomes in certain people.


The study results demonstrated that the SARS-CoV-2 Omicron BA.2.2 variant spread swiftly with a doubling time of 3.4 days in Hong Kong, rapidly exceeding test-and-trace, isolation, and quarantine capacity, resulting in a significant number of COVID-19 patients in need of hospitalization with limited resources. This was despite strict public health and social precautions. The comparatively shorter generation time and serial interval of the Omicron BA.2 sublineage relative to the prior SARS-CoV-2 variants and the enhanced inherent transmissibility led to the rapid viral spread in the population.

The drastic COVID-19-linked deaths in Hong Kong during the fifth wave could be ascribed to the rapid spread of infection and low vaccination coverage among older persons. While total vaccine coverage was 70% at the beginning of the fifth wave, only 50% and 20% of those aged ≥65 and ≥80 years had completed the main series of vaccinations, respectively. Up to March 23, 2021, 92.5% of all SARS-CoV-2-related deaths with age recorded occurred in those aged ≥65 years and 70.8% in people aged ≥80.

In the early portion of the fifth wave, the researchers discovered an identical fatality risk for unvaccinated COVID-19 patients relative to prior waves, implying that BA.2's inherent severity may not be significantly lower than the original SARS-CoV-2 strain. In addition, the authors reported that Omicron BA.2 infection in people aged 65-79 years or above 80 years with a complete primary vaccination status had a considerably reduced probability of death.

Given the lack of information on the type of SARS-CoV-2 variant for all individual cases and the possibility that a small number of COVID-19 cases, including mortality from clusters of the SARS-CoV-2 Delta infections, were included in the study, the present CFR estimates may fractionally overestimate the fatality risk of Omicron in Hong Kong.


The study findings suggested that the SARS-CoV-2 Omicron BA.2.2 strain harbored a shorter generation time, incubation period, and serial interval than other viral variants. In Hong Kong, the Omicron variants, notably the BA.2 sublineage, were shown to have a superior transmission potential. Individuals with an incomplete primary vaccination regimen, when infected with Omicron, experienced an equivalent mortality risk as those who got infected with the SARS-CoV-2 ancestral strain in the prior COVID-19 pandemic waves.

Overall, the present data underscored the need for high SARS-CoV-2 vaccination coverage, particularly among older persons, and the necessity to reevaluate public health and social strategies in epidemic management in response to a substantially transmissible viral variant in the future.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
  • Mefsin, Y. et al. (2022) "Epidemiology of infections with SARS-CoV-2 Omicron BA.2 variant in Hong Kong, January-March 2022". medRxiv. doi: 10.1101/2022.04.07.22273595.

Posted in: Medical Science News | Medical Research News | Disease/Infection News

Tags: Coronavirus, Coronavirus Disease COVID-19, covid-19, Epidemiology, Immunization, Mortality, Omicron, Pandemic, Public Health, Respiratory, Ribonucleic Acid, SARS, SARS-CoV-2, Severe Acute Respiratory, Severe Acute Respiratory Syndrome, Syndrome, Vaccine

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Shanet Susan Alex

Shanet Susan Alex, a medical writer, based in Kerala, India, is a Doctor of Pharmacy graduate from Kerala University of Health Sciences. Her academic background is in clinical pharmacy and research, and she is passionate about medical writing. Shanet has published papers in the International Journal of Medical Science and Current Research (IJMSCR), the International Journal of Pharmacy (IJP), and the International Journal of Medical Science and Applied Research (IJMSAR). Apart from work, she enjoys listening to music and watching movies.

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