The influenza bug circulating in Hong Kong is a normal human flu strain, but seems to be more virulent, said Dr Lim Poh Lian, a senior infectious disease consultant at Tan Tock Seng Hospital.

This happens every few years as the flu bug changes a little, she explained. People who catch the more virulent bugs suffer more severe symptoms or find it takes longer to recover.

But if you are not travelling, it might be a good idea to wait until next month when a new vaccine comes out, Dr Lim suggested, as that might be a better match for the flu strains that are now circulating.

All flu vaccine protects against three strains, and these are changed every six months, depending on the bugs that are expected to be most active.

Both the current and new vaccine will give some protection, the Ministry of Health said, even if neither gives full protection because there are more strains of the virus in play now than the vaccine can protect against.

Annual flu shots are recommended for the very young, the elderly and those with chronic ailments such as diabetes or heart diseases. Pregnant women in their second or third trimester should also get vaccinated as they also are at higher risk of getting more sick should they catch the virus.

But the ministry added that there is no rush to get the flu jab simply because of what is happening in Hong Kong.

The health authorities there have ruled out both Sars and avian flu as the cause of the deaths, the Health Ministry said, adding that it was in touch with its counterparts in Hong Kong.

The ministry stressed good hygiene practices like washing hands regularly and thoroughly, and wearing a mask if unwell. Anyone with flu-like symptoms - fever, cough, sore throat and runny nose - should see a doctor and stay away from public areas and their work place until fully recovered.

Sars, or severe acute respiratory syndrome, which hit Singapore almost exactly five years ago, infected 238 people here and killed 33. In the eight months it raged, Sars killed 349 people in China and 299 in Hong Kong.

Symptoms for Sars were similar to those for the flu - high fever, headache and, in the young, diarrhoea and vomiting.

But even the normal human strains of flu can kill. The Health Ministry said that up to 500 million people worldwide catch the flu each year, with up to 500,000 deaths.

Singapore does not track deaths caused by flu. But the ministry said there has been no unusual increase in upper respiratory tract infections, which include both the flu and the common cold.

Last week, 13,000 people were treated at polyclinics for these symptoms - about 2,000 fewer than for the same week last year.

Nonetheless, more than 120 clinics, especially those dealing with young children, ordered more flu vaccine yesterday in anticipation of heavy demand following the Hong Kong outbreak, according to the drug companies supplying the flu shots.

Paediatrician Chan Sing Kit, one of the doctors who ordered more vaccine yesterday, said she is expecting parents of young kids to get their children vaccinated before returning to school next week after the week-long break.

The Education Ministry said yesterday that schools will be more vigilant when they reopen on Monday, and monitor students who are unwell.

'Students who are unwell should consult a doctor. Parents are also advised not to send their children to school if they are unwell,' it said.

salma@sph.com.sg