Impact of Influenza in South-East Asia

ABSTRACT

Influenza infection is asymptomatic in up to 75% of cases, but outbreaks result in significant morbidity. Reports found that severe influenza complications tend to occur among the very young (<5 years) and very old (>65 years), especially those with underlying co-morbidities like diabetes mellitus and heart disease. Even with no co-morbidity, some older persons with severe influenza may require hospitalisation or intensive care, with increased risk of myocardial infarction and stroke. In South-East Asia, influenza was often seen as a mild problem and was not deemed notifiable until the appearance of the Influenza A(H1N1) pandemic in 2009. For decades the data made available were based on extrapolated estimates collected mainly from paediatric populations, resulting in inconsistent findings. Following expanded surveillance across the region using national surveillance systems for influenza-like illness (ILI) and severe acute respiratory illness (SARI), and better diagnostic methods, improved estimates of disease burden was achieved in South-East Asia. However, two studies conducted in 2008-2010 reported findings ranging from 2-3% to 11%. With regards to increased risk of complications, the estimated global annual attack rates for influenza were 5-10% in adults and 20-30% in children, resulting in 3-5 million cases of severe illness and 290,000-650,000 deaths. A study In Singapore reported that influenza is associated with annual excess mortality rates (EMR) of 11-14.8 per 100 000 person-years, especially affecting the elderly;these rates are comparable to that of the USA. As for hospitalisation rates of children under 5 years with seasonal influenza, the USA estimated a rate of 1.4 per 100,000. Comparable rates were reported in Singapore (0.7-0.9), Thailand (2.4), Viet Nam (3.9-4.7), and the Philippines (4.7). In 2018, an updated study reported a mean annual influenza-associated respiratory EMR of 4.0-8.8 per 100 000 individuals, with South-East Asia showing a high mortality rate of 3.5-9.2 per 100,000 individuals. It was already estimated in Thailand in 2004 that influenza resulted in USD23-63 million in economic costs, with the main contribution from lost productivity due to missed workdays. Thus, comparable to countries in temperate climate, the clinical and socioeconomic impact of influenza in South-East Asia appear to be just as substantial. With the emergence of the COVID-19 pandemic in 2020, global influenza incidence dropped dramatically. In South-East Asia, the trend in influenza detections was similar to the rest of the world, with numbers slightly higher than average in early 2020, followed by a quick drop-off by the end of April 2020. After April 2020, the detection rate remained low until late July 2020, when Influenza A(H3N2) predominated in Cambodia, Malaysia, the Philippines, Singapore, Thailand and Timor-Leste;influenza B in Lao People's Democratic Republic but with an upsurge in A(H3N2) activity. Following a two-year hiatus, influenza outbreaks began to re-emerge significantly since early 2022. From February through August 2022, influenza activity in the southern hemisphere remained lower than in pre-COVID-19 pandemic years, but was at the highest level compared to similar periods since the start of the COVID-19 pandemic. Reasons for the reduction during the COVID-19 pandemic include non-pharmaceutical interventions (NPIs), reduced population mixing and reduced travel, and possibly viral interference between SARS-CoV-2 and influenza virus in the same host. In general, the reduction in influenza detections however does not appear to be associated with lack of testing. The World Health Organisation (WHO) continues to recommend that vaccination is the most effective way to prevent infection and severe outcomes caused by influenza viruses. Although influenza vaccine is not commonly used in most countries in South-East Asia, its burden is similar in other parts of the world where influenza vaccine is now routinely used. Currently, the countries in South-East Asia that are providing free influenza vacc na ion for those at high risk include Thailand, Singapore, the Philippines and Lao People's Democratic Republic.Copyright © 2023