HAS Influenza A (H1n1) run its course? It is on the wane, said the Ministry of Health, when it lowered the yellow alert to green nearly two weeks ago. Time for some relief, but probably not yet celebration. Elsewhere, infections have not returned to levels normally seen for seasonal influenza. Nor has it stopped infecting people that usually do not catch the flu. Previous pandemics, including the 1918 Spanish flu, packed a much deadlier second wave that killed younger and healthier people as well as the sick and elderly. The H1N1 pandemic has yet to peak, 10 months after the virus surfaced in Mexico and spread around the world.

Epidemiology not being an exact science, the prudent approach is to refrain from letting down the guard completely. The ministry continues to monitor the situation, even as acute respiratory infections remain below epidemic level since last August. Fear of a spike as students returned to school last month proved unfounded. The good news is, H1N1 has not been as deadly as Sars. The even better news, from the public health perspective, is that people have responded as robustly, even though the virus turned out to be a less serious threat. More than 420,000 got themselves vaccinated after the shots became available last November. An estimated 415,000 the virus is presumed to have infected now have immunity.

The remaining gains are in hygienic habits formed. Regular hand washing, sneezing etiquette and personal cleanliness are sensible measures that can prevent not only H1N1 but many other types of infection also, There is no assurance that the virus will never return in a more virulent form. So, Health Minister Khaw Boon Wan offers eminently sound advice to stay vigilant: "Colour change does not mean the end of H1N1." Neither should anyone complain about the inconvenience of long queues for temperature screening and other preventive measures even though, in retrospect, the danger turned out to be not as big as initially feared.

The virus is still out there. It could mutate into a more dangerous strain. Or it could mix with the more deadly bird flu virus, which remains endemic in poultry in many Asian countries. It is unclear what changes, if any, it will undergo during the coming southern winter. Neither is it possible to rule out untoward consequences as it starts to spread to sub-Saharan Africa. There, poverty and lack of sanitation, immunisation and therapy pose huge preventive and mitigating obstacles. Inimuno-compromised people with high rates of HIV, tuberculosis and other chronic infections are at risk. So, even as it recedes to the background in Singapore, H1N1 bears constant and careful watching.