We have to watch the virus closely. Scientists across the globe are working to develop a vaccine for the flu that would be effective during the second wave.'

The ICGEB is an international organisation involved in advanced research and training in molecular biology and biotechnology.

Senior epidemiologist and executive director of the Indian Clinical Epidemiology Network (IndiaCLEN) Narendra Arora said no one at the moment knows 'how lethal the virus could be in future'.

IndiaCLEN is a cluster of seven regional clinical epidemiology units that are members of the International Clinical Epidemiology Network (INCLEN).

'It is the history of the flu pandemics that is making scientists believe that the second wave of the flu could be deadly. Flu pandemics historically come in waves, often getting worse as they go and there is a likelihood that swine flu may come back in a more severe form,' added Arora.

Citing the example of the 1918-19 Spanish flu pandemic, which was considered the worst of all time that had left 40 million people dead, Arora said: 'The Spanish flu started with a mild wave followed by a deadly second wave later in the year,' he said.

According to experts, surveillance is important to halt the mass spread of the virus and the country needs to prepare a database of the flu-affected people.

'Every season 10 percent of the population is affected with common flu, and if the number in the coming season rises to 25-30 percent then we need to worry. We have to have a database of the people affected with swine flu in the country to check against any unusual break,' said Naresh Gupta, senior doctor at Maulana Azad Medical College in Delhi.

The swine flu pandemic is the first since the Hong Kong flu pandemic in 1968, which killed one million people worldwide.

India reported the first swine flu case on May 16. The World Health Organisation (WHO) raised its alert against swine flu to the highest level - Phase 6.

India has reported 558 swine flu cases, of which 470 have been discharged from the identified health facilities till Monday. A total of 2,479 people have been tested for the flu so far.