Impact assessment of non-pharmaceutical interventions against coronavirus disease 2019 and influenza in Hong Kong: an observational study

Abstract

Background: A range of public health measures have been implemented to suppress local transmission of coronavirus disease 2019 (COVID-19) in Hong Kong. We examined the effect of these interventions and behavioural changes of the public on the incidence of COVID-19, as well as on influenza virus infections, which might share some aspects of transmission dynamics with COVID-19.

Methods: We analysed data on laboratory-confirmed COVID-19 cases, influenza surveillance data in outpatients of all ages, and influenza hospitalisations in children. We estimated the daily effective reproduction number (R_t) for COVID-19 and influenza A H1N1 to estimate changes in transmissibility over time. Attitudes towards COVID-19 and changes in population behaviours were reviewed through three telephone surveys done on Jan 20-23, Feb 11-14, and March 10-13, 2020.

Findings: COVID-19 transmissibility measured by R_t has remained at approximately 1 for 8 weeks in Hong Kong. Influenza transmission declined substantially after the implementation of social distancing measures and changes in population behaviours in late January, with a 44% (95% CI 34-53%) reduction in transmissibility in the community, from an estimated R_t of 1·28 (95% CI 1·26-1·30) before the start of the school closures to 0·72 (0·70-0·74) during the closure weeks. Similarly, a 33% (24-43%) reduction in transmissibility was seen based on paediatric hospitalisation rates, from an R_t of 1·10 (1·06-1·12) before the start of the school closures to 0·73 (0·68-0·77) after school closures. Among respondents to the surveys, 74·5%, 97·5%, and 98·8% reported wearing masks when going out, and 61·3%, 90·2%, and 85·1% reported avoiding crowded places in surveys 1 (n=1008), 2 (n=1000), and 3 (n=1005), respectively.

Interpretation: Our study shows that non-pharmaceutical interventions (including border restrictions, quarantine and isolation, distancing, and changes in population behaviour) were associated with reduced transmission of COVID-19 in Hong Kong, and are also likely to have substantially reduced influenza transmission in early February, 2020.

Funding: Health and Medical Research Fund, Hong Kong.

Figure 1

COVID-19 cases in Hong Kong by date of reporting (A) and date of symptom onset (B) The Chinese New Year, a major winter festival in Hong Kong, was on Jan 25, and there were public holidays on Jan 25–28. Most schools started holidays on Jan 22 and were scheduled to resume on Feb 3. The Hong Kong Government has deferred class resumption several times and closures are now until further notice without an expected resumption date. 94 asymptomatic cases are not shown in panel B. All dates are in 2020. COVID-19=coronavirus disease 2019.

Figure 2

Incidence and transmissibility of COVID-19 in Hong Kong (A) Incidence of local COVID-19 cases in Hong Kong (dark blue bars) and cases infected overseas but detected locally (light blue bars). Augmented incidence includes estimated additional cases that have occurred but have not yet been identified due to reporting delays. (B) Estimates of the daily R_t of COVID-19 over time. The pink shaded area indicates 95% CIs. The dashed line indicates the critical threshold of R_t=1. All dates are in 2020. COVID-19=coronavirus disease 2019. R_t=effective reproduction number.

Figure 3

Incidence, hospitalisation rate, and R_t of influenza A (H1N1) in 2019–20 (A) Weekly incidence, calculated as the weekly consultation rate multiplied by the proportion of laboratory specimens testing positive for influenza A (H1N1). (B) Daily hospitalisation rate with influenza A (H1N1) in children in two large hospitals in Hong Kong. (C) Estimated R_t in Hong Kong based on the influenza proxy data, with 95% CIs indicated by the pink shaded region. (D) Estimated R_t in Hong Kong based on the hospitalisation data, with 95% CIs indicated by the pink shaded region. We stopped estimating R_t when the local epidemic ended, indicated by a reduction in influenza proxy to very low levels and no further influenza hospitalisations in children. Dashed lines indicate the critical threshold of R_t=1. Shaded bars show the dates of Chinese New Year (light blue) and school closures (grey). R_t=effective reproduction number.

Figure 4

Incidence and R_t of influenza in 2010–11 and 2014–15 Weekly incidence of influenza (all type and subtypes) was calculated as the weekly consultation rate multiplied by the proportion of laboratory specimens testing positive for influenza in the winter influenza season of 2010–11 (A) and 2014–15 (B). R_t in Hong Kong was estimated based on the influenza proxy data, with 95% CIs indicated by the pink shaded region, for the winter influenza season of 2010–11 (C) and 2014–15 (D). Dashed lines indicate the critical threshold of R_t=1. Shaded bars show the dates of Chinese New Year (light blue) and school holidays (grey). R_t=effective reproduction number.