If you think the worst of the influenza A H1N1 is now a thing of past, you are mistaken.

We are still in the midst of the first wave of the pandemic and experts warn that a second wave may be coming any time soon as winter sets in.

The World Health Organization (WHO) has warned that countries in the region - particularly India and Thailand - could see a second wave of the pandemic influenza this winter, which could be much more severe than the first attack.

"We have learnt from experience that influenza pandemics have a tendency to attack populations in periodic waves and that the second or third wave may cause more severe morbidity and mortality than the first," Dr Samlee Plianbangchang, the South-East Asian regional director of the WHO, warned countries in the region.

The influenza cases are already seeing a rising trend across India in the past two weeks. The number of laboratory- confirmed cases - which is a fraction of the actual number of cases in Delhi - have risen from 20 to 25 a day in the beginning of November to 65 to 70 cases a day now.

One of the reasons for this rise is attributed to the influx of cases referred from neighbouring states to Delhi, officials said. "There are no signs of a second wave as yet. Even in cold Scandinavian countries, there is no second wave till now. But the level of alertness continues," a health ministry official said.

"We are still in the middle of the first wave. The actual impact of the pandemic has not been fully determined," said Dr Jai P. Narain, director of communicable diseases at the WHO. "We must anticipate and prepare for the second wave, which may come soon." The experience of the first wave so far has given important insights into the nature of the virus and the pandemic. The first is communicability or transmissibility. It has now been established that the novel influenza virus is highly communicable and spreads fast. The second factor is virulence or the severity of the illness the virus causes.

"This is one area where we are still not very sure. So far, we have seen that the illness the virus causes is mild to moderate - one to two per cent patients need hospitalisation and fatalities are even less. But this may change in the future," said Narain. The third critical factor is the health systems' capacity to be able to respond.

Even if the current pattern of normally mild illness continues, the impact of the pandemic during the second and third waves could worsen as larger numbers of people could get infected and more severe symptoms might appear.

People who become severely ill will require intensive care, creating huge burden on health services, pressures could overwhelm ICUs and disrupt care for other diseases. That's why it is necessary to improve 'surge capacity' of hospitals.

The winter surge would be different from the situation in the ongoing first wave, needing a different strategy to tackle it. "The first wave was mostly imported, so airport vigilance was good enough. Now, the virus is circulating in the community and it would be much more difficult to control its spread," a health official said.

"Now that the infection is widespread within the country, the role of airport screening is not relevant anymore," said Narain.

Also, testing may not be necessary for everyone coming with the symptoms, as it puts a huge burden on the health system. Narain felt the decision whether a patient should be put on antiviral treatment, without testing, should be left to the judgment of the doctor.

While measures such as personal hygiene, hand washing and social distancing still need to be taken with vigour, the focus is now going to be on the availability of a vaccine to prevent H1N1 infection.

As far as treatment of those infected is concerned, Oseltamivir (sold under the brand name of Tamiflu) remains the first choice.

The WHO has revised its guidelines for the use of this drug.

The UN agency, in its latest communication to the health ministry, recommended that all those in 'at-risk' groups should be treated with antivirals as soon as possible if they have flu symptoms. This includes pregnant women, children under two years old, and people with underlying conditions such as respiratory problems.

Secondly, people who are not in the 'at-risk' group but who have persistent or rapidly worsening symptoms should also be treated with antivirals. These symptoms include difficulty in breathing and high fever lasting more than three days.

Thirdly, people who have developed pneumonia should be given both antivirals and antibiotics, as it has been observed that in many severe cases of H1N1 cases bacterial infection also develops.

Most developed countries have begun vaccination and want to immunise people before the second wave comes. Indian vaccine manufacturers have indicated they will have the vaccine ready for use by March or April 2010.

But enough doses of vaccine would not be available for everyone to be immunised nor does the country have resources to do that. So, India would have to prioritise which set of population it should be given to. Healthcare workers would be the first to be vaccinated. Then it could be given to population groups which are vulnerable - very young, very old and those with underlying conditions.

Till the indigenous vaccines become available, the WHO has promised to make available imported vaccine for frontline health workers in India. All foreignmanufactured vaccines will be allowed to be used in India only after they undergo necessary 'bridging' clinical trials in the country, the health ministry clarified.