Unsynchronized influenza epidemics in two neighboring subtropical cities.

OBJECTIVE: The aim of this study was to examine the synchrony of influenza epidemics between Hong Kong and Shenzhen, two neighboring subtropical cities in South China. METHODS: Laboratory-confirmed influenza data for the period January 2006 to December 2016 were obtained from the Shenzhen Center for Disease Control and Prevention and the Department of Health in Hong Kong. The population data were retrieved from the 2011 population censuses. The weekly rates of laboratory-confirmed influenza cases were compared between Shenzhen and Hong Kong. RESULTS: Unsynchronized influenza epidemics between Hong Kong and Shenzhen were frequently observed during the study period. Influenza A/H1N1 caused a more severe pandemic in Hong Kong in 2009, but the subsequent seasonal epidemics showed similar magnitudes in both cities. Two influenza A/H3N2 dominant epidemic waves were seen in Hong Kong in 2015, but these epidemics were very minor in Shenzhen. More influenza B epidemics occurred in Shenzhen than in Hong Kong. CONCLUSIONS: Influenza epidemics appeared to be unsynchronized between Hong Kong and Shenzhen most of the time. Given the close geographical locations of these two cities, this could be due to the strikingly different age structures of their populations.

Anti-phase synchronization of influenza A/H1N1 and A/H3N2 in Hong Kong and countries in the North Temperate Zone.

OBJECTIVES: Influenza affects 10% of the global population annually. Hong Kong is a subtropical city close to the North Temperate Zone (NTZ). Influenza-associated mortality rates in Hong Kong were 27.53, 79.55, 36.94, and 48.72 per one million person-years in 2014, 2015, 2016, and January to July 2017, respectively. The aim of this study was to explore the temporal patterns of influenza A subtypes in both Hong Kong and the NTZ. METHODS: Weekly reported cases of influenza A/H1N1 and A/H3N2 from January 2010 to July 2017 were downloaded from the Centre for Health Protection in Hong Kong and FluNet (provided by the World Health Organization). The spatio-temporal patterns of A/H1N1 and A/H3N2 in Hong Kong and the NTZ were explored. Anti-phase synchronization between A/H1N1 and A/H3N2 is indicated by a high for A/H1N1 coinciding with a low for A/H3N2, and vice versa. RESULTS: Anti-phase synchronization was evident in both Hong Kong and the NTZ. It was found that A/H3N2 clearly dominated in the 2011/2012, 2014/2015, and 2016/2017 influenza seasons. CONCLUSIONS: These results are novel in identifying anti-phase synchronization in influenza A subtypes in Hong Kong and the NTZ. These findings should inform public health preparedness for future epidemics of A/H3N2, which are typically more severe than those of A/H1N1.