H1N1 appears milder than seasonal influenza in Singapore

In the tropical climate of Singapore, influenza A (H1N1) was less likely to cause upset stomach and fever and appeared milder than the seasonal flu, according to a recent study.

Researchers from the National University Hospital in Singapore analyzed the symptoms of 2,683 individuals tested with a rapid-detection assay between May 2009 and July 2009. Of these, 742 (27.6%) tested positive for any type of influenza, including 547 cases (20.4%) of H1N1.

Early H1N1 cases had a different symptom pattern and appeared somewhat milder than the seasonal flu. H1N1 patients most commonly had cough (88.1%), fever (79.3%), sore throat (53.7%) and runny nose (49.9%). Patients with seasonal flu had fever (88%), cough (81.4%), runny nose (55.7%) and sore throat (38.3%).

The pandemic virus was more likely to affect children and young adults and avoided the elderly, whereas the seasonal flu affected patients of all ages, with a higher incidence in children 5 years and younger.

Influenza may be least likely to survive in tropical climates such as that of Singapore, according to background information supplied by the researchers. In Singapore, temperatures range from 73·F to 95·F, with humidity from 48% to 100%. Flu cases peak in June through July and November through January.

“It is likely that these symptom patterns will continue to evolve and change as the novel pandemic influenza A (H1N1/2009) eventually predominates, globally, in the susceptible human population,” the researchers wrote. “However, this early window period in this first wave of the pandemic has provided an opportunity to compare the symptomatology of these different influenza viruses in this particular tropical environment and ethnically diverse population during this transitional period.”

Tang JW Arch Intern Med. 2010;170:861-867.