Influenza-associated hospitalization in a subtropical city

A new study shows that tropical and subtropical countries suffer far more illness and death during flu outbreaks than previously imagined, with both hospital admissions and deaths rising considerably during a flu outbreak.

Most strains of influenza are successfully fought off by the vast majority of people, who are back to normal within a week or two. Nevertheless, flu can cause serious illness, and sometimes death, in the elderly and other vulnerable people. A flu outbreak also puts a considerable strain on hospitals, as the number of people admitted always increases during an outbreak, not just for respiratory problems but for a variety of other medical conditions.

In cooler ('temperate') countries, all this has been known-backed up by a number of statistical studies-for many years. However, it has been assumed that flu is only a minor problem for people in the warmer parts of the world - the tropics and subtropics. Most countries in these regions are developing nations, and collecting and analysing data on flu and its complications is not easy. Flu outbreaks also happen at unpredictable and irregular intervals, in contrast to the seasonal pattern seen in temperate countries, and this creates problems for the statisticians.

Researchers in Hong Kong (which is in the subtropics) realised that they were well placed to study the impact of flu in such a location. Hong Kong has a sophisticated health care system, which includes advanced computerised record keeping. Ninety-five per cent of people admitted to hospital are treated in public hospitals and their records were available to the researchers. They developed new statistical methods to allow for the irregular nature of Hong Kong's flu outbreaks.

The researchers found that, during outbreaks, hospital admissions increased, not just for respiratory diseases such as pneumonia but also for heart conditions, stroke and diabetes. The increases were most noticeable for older people. Overall, influenza was responsible for 11.6% of admissions for respiratory disease, 1.5% of admissions for stroke, 1.8% of admissions for heart attacks, and 3.5% of admissions for diabetes. These figures are comparable with what has been found in temperate countries, for example the USA. It has also been noted, however, that more children are admitted to Hong Kong's hospitals during flu outbreaks than happens in the US.

Hong Kong is a wealthy subtropical city and it is different in many respects from low-income countries in the tropics, which face massive problems with other diseases and lack the modern health care that is available in Hong Kong. Nevertheless, the results of this study suggest that influenza deserves to be given a higher priority than it has at present in the tropics and subtropics. The authors urge the introduction of vaccination programs for people at high risk, particularly the elderly.

http://www.plosmedicine.org