Shifts in Population Immunity and Public Health Measures Impact Epidemic Dynamics in Hong Kong

Strategic Insights -

The rapid evolution of SARS-CoV-2 and population-level vaccine administration has significantly shifted the population immunity in Hong Kong, leading to a deeper understanding of how population immunity and public health and social measures (PHSMs) have shaped the epidemic dynamics across age groups within the population. Researchers from the University of Hong Kong have developed an age-structured, multistrain model to estimate key parameters including transmissibility for the emerging BA.5 subvariant and the effects of PHSMs on transmission.

Key Takeaways:

  • The rapid evolution of SARS-CoV-2 and population-level vaccine administration have significantly shifted the population immunity in Hong Kong.
  • The emergence of Omicron BA.5 with a strong ability of immune evasion necessitates a deeper understanding of how population immunity and PHSMs have shaped the epidemic dynamics across age groups within the population.
  • Financial supporters for this research include Theme-based Research Scheme, General Research Fund, Hong Kong Research Grants Council, RGC Senior Research Fellowship, AIR@innoHK program of the Innovation and Technology Commission of the Hong Kong SAR Government.
  • The research found that reactive PHSMs implemented in February 2022 decreased the effective reproductive number without the effect of immunity (R-t(WI)) by 67% (95% credible interval [CrI], 52%-78%).
  • Subsequent relaxation of control measures since April 2022 drove R-t(WI) back to 3.4 (95% CrI, 2.8-4.1) by late May.
  • Prior to the fifth wave, only 15% of the Hong Kong population had immunity and protection against BA.2 infection, which increased significantly to 55% within 2 months given 47% cumulative infections and 30% vaccination uptake.
  • The emergence of BA.5 led to a 15% decrease in population immunity against BA.2 infection during the end of May.
  • The research highlighted the potential risks posed by immune-evasive variants in the context of waning immunity and control relaxation.

Statistics:

  • The effective reproductive number without the effect of immunity (R-t(WI)) decreased by 67% (95% credible interval [CrI], 52%-78%) after implementing reactive PHSMs in February 2022.
  • R-t(WI) drove back to 3.4 (95% CrI, 2.8-4.1) by late May after relaxing control measures since April 2022.
  • 15% of the Hong Kong population had immunity and protection against BA.2 infection prior to the fifth wave.
  • 55% of the Hong Kong population had immunity and protection against BA.2 infection within 2 months after 47% cumulative infections and 30% vaccination uptake.
  • Population immunity against BA.5 infection was 15% lower than that against BA.2 during the end of May.

Sources:

  • NewsRx. New COVID-19 Study Findings Reported from University of Hong Kong (Impact of Population Immunity and Public Health Measures On the Transmission of Omicron Subvariants Ba.2 and Ba.5 In Hong Kong). Medical Letter on the CDC & FDA. July 6, 2025; p 125.
  • The Journal of Infectious Diseases. Impact of Population Immunity and Public Health Measures On the Transmission of Omicron Subvariants Ba.2 and Ba.5 In Hong Kong. Oxford Univ Press Inc, Journals Dept, 2001 Evans Rd, Cary, NC 27513, USA.
  • University of Hong Kong. World Hlth Org Collaborating Ctr Infect Dis Epidem, Li Ka Shing Faculty of Medicine, School of Public Health, Hong Kong, People's Republic of China.