The health authorities say, however, that these side effects are similar to those experieced by patients who have been given the vaccine for the seasonal flu.

The 12 patients, aged between 24 and 58, were flagged by doctors as either needing hospitalisation or having  reactions considered life threatening. They have all since recovered or are recovering.

They were among 108 people who suffered adverse effects in one form or another, suspected to have resulted from the H1N1 vaccine, said a Health Sciences Authority (HSA) spokesman.

The rest had milder reactions - itchiness, swollen eyelids, sore throat, fever, diarrhoea and dizziness, which usually surfaced on the day of the jab and cleared up in a couple of days.

The Health Ministry says more than 240,000 doses of the H1N1 vaccine have been administered here, compared to the usual demand of 400,000 doses of the seasonal flu vaccine.

Only one case of serious reaction to the seasonal flu vaccine was reported last year and three the year before, but the HSA said a lack of awareness could have led to under-reporting by doctors with the H1N1 vaccine, doctors have been asked to report all cases of reactions to the jab.

The HSA spokesman added that the adverse effects from the H1N1 jabs are consistent with findings by the World Health Organisation - that the pandemic vaccine triggers reactions similar to those of vaccines for the seasonal flu.

The dozen patients with reactions assessed by their doctor to be serious had symptoms including puffy eyes, flushing, breathing difficulties from a drop in blood pressure and hypersensitive reactions involving skin and joints.

One patient had dystonia, an involuntary jerking of limbs; another was wracked by persistent vomiting.

Five others, including a patient with an upper respiratory tract infection and difficulty in breathing, are still being monitored, said the HSA spokesman.

Four of the 12 patients had pre-existing medical conditions such as asthma or allergies to certain medications.

The HSA spokesman pointed out that, other than for the five patients with allergic reactions, it cannot be concluded that the vaccines caused the serious effects for the others; the symptoms they had could have arisen coincidentally, or from an underlying disease.

Agreeing, Associate Professor Paul Ananth Tambyah, who heads the National University Hospital's infectious diseases division said the reactions could be due to medical reasons or might have coincided with the vaccination date.

In the Northern hemisphere, the number of H1N1 virus infections appears to have peaked and is starting to dip.

Two major mutations to the virus have been reported worldwide so far, even as doctors here look for signs of more: One is resistant to the anti viral-drug Tamiflu, and the other is associated with a more severe form of the H1N1 flu.

Prof Tambyah said both mutations should be covered by the vaccine.

Overall, hospital admissions here are now low - about 10 to 15 cases, down from 74 at the peak of the H1N1 outbreak in August. Polyclinics have treated about 13,000 patients with upper respiratory tract infections each week, down from 24,477 a week in August.

But with people returning from holidays abroad and schools re-opening soon, the numbers could go up again.

With up to a third of Singaporeans estimated tryhave been infected with the virus in the first wave, the second wave could be the same or smaller, said Prof Tambyah. This is because once half the people are hit, the virus, finding fewer people to infect, will naturally die out.

The Health Ministry yesterday repeated its call for good hygiene habits. It also urged parents to get H1N1 shots for their children and to keep them at home if they are unwell when schools re-open next week.

Some side effects

• Puffy eyes
• Flushing
• Breathing difficulties
• Hypersensitive reaction involving skin and joints
• Dystonia, an involuntary jerking of the limbs
• Vomiting
• Milder reactions like itchiness, swollen eyelids, sore throat, fever, diarrhoea and dizziness