Symptom patterns differ between pandemic and seasonal influenza

In a tropical environment, influenza A(H1N1) appeared milder than seasonal flu, was less likely to cause fever and upset stomach and more likely to infect younger individuals, according to a new report.
In 2009, a new rapid detection method was designed to detect genes from the new influenza A(H1N1) virus and was used during a period of enhanced surveillance for influenza after global pandemic alerts were announced.

Against such a background, doctors analysed trends and symptoms among patients in Singapore during this period, from May to July 2009.
During the 12-week surveillance period, 2,683 individuals with symptoms, close contacts or travelers were tested using the rapid-detection assay. Of these, 742 (27.6 per cent) were positive for any type of influenza virus, including 547 cases (20.4 per cent) of influenza A(H1N1).
Early pandemic influenza cases appeared slightly milder than seasonal flu and had a different symptom pattern. The most common symptoms among individuals with pandemic influenza were cough (88.1 per cent), fever (79.3 per cent), sore throat (53.7 per cent) and runny nose (49.9 per cent). Individuals with the predominant strain of seasonal influenza most commonly had fever (88 per cent), cough (81.4 per cent), runny nose (55.7 per cent) and sore throat (38.3 per cent).
Seasonal influenza affected individuals of all ages, with a higher proportion of those five years and younger, while the pandemic virus was more likely to affect children and young adults and had very few elderly cases.
“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 authors concluded from the findings.
“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.”
Archives of Internal Medicine 2010;170:861-867