At a time when the World Health Organisation (WHO) is being criticised for over-reacting and declaring a pandemic of the basis of only geographical spread of a novel flu virus, a leading virologist has argued that severity should not be included as a criterion.

Long before WHO's declaration, there was no doubt that this was a pandemic, observed J.S. Malik Peiris. He leads a multi-disciplinary research programme at the University of Hong Kong, studying emerging viral diseases, including influenza, which spread from animals to humans.

“It was a new virus, which we didn't know about, and [it] swept across the world,” he pointed out at a recent conference on the current pandemic organised by the University of Hong Kong's Journalism and Media Studies Centre.

“If it had been just marginally more severe, we would have been shouting a different tune right now,” he remarked when this correspondent met him after the conference.

Of Sri Lankan origin, Dr. Peiris is widely known for his key role in the discovery of the virus responsible for Severe Acute Respiratory Syndrome (SARS), a new disease that suddenly manifested in 2002 and caused global panic by its rapid spread, infecting and killing people in over 30 countries. A Fellow of the Royal Society, he ranks as one of the most highly cited scientists.

It was extremely difficult to measure the severity of a flu pandemic, he observed during a presentation at the conference. “Even up to now, we really don't have good assessments of severity.” The data was not there in the early phase of present pandemic to say that it was mild. There was also the issue of how 'mild' would be defined.

“So it would completely paralyse international public health policy, I think, if severity is linked to the definition of a pandemic.”

In the event of a pandemic, the message that needs to be conveyed to the public is that not all pandemics are severe. Some, like the one in 1968, can be extremely mild.

Influenza is probably the only infection that can emerge and spread worldwide in a matter of months, he told The Hindu. For such a global impact, the virus has probably to be transmitted via the respiratory tract. Although there are other viruses, such as measles, respiratory syncytial virus and parainfluenza viruses, that are spread by the respiratory route, “none of those have the capacity to reinvent itself and come back as a big variant” the way influenza can.

During the interview, Dr. Peiris emphasised the importance of monitoring viruses circulating in animals, especially in livestock. Pandemics have been caused by flu viruses that came from animals. It was the flu viruses common in domestic livestock that were the most likely to cross over into humans.

The pandemics of 1957 and 1968 – as well as the current one – were produced by flu viruses that had undergone reassortment. (If two different flu viruses infect the same cell, they can swap genes, a process known as reassortment. Such reassortment can lead to novel flu viruses being produced.)

It has been believed the pandemic of 1918 was caused by a purely avian virus that had mutated and thus become adapted to humans. But that view was questioned in a paper published last year by a team of scientists that included Dr. Peiris. In the paper, which appeared in the Proceedings of the U.S. National Academy of Sciences, the scientists dated the origins of the various genes of the 1918 virus.

The evolutionary dating of the 1918 virus suggested that it was in some other mammalian host for some period of time before the pandemic emerged, said Dr. Peiris. “On the basis of our data …. we think the most probable scenario is that the virus obviously came from avians but it adapted in some other host and then caused the pandemic.” Pigs may well have been that host.

In a recent paper published in the journal Science, Dr. Peiris and colleagues reported that the pandemic H1N1 virus, along with a swine virus, had produced a reassortment in pigs. The reassorted virus was not likely to be very dangerous because only one of its genes came from the pandemic virus, he said. But other reassortments could occur. In their paper, the scientists advocated greatly increased surveillance of flu viruses in swine.

Before the current pandemic began, there were fears that it could be caused by the H5N1 bird flu virus. But Dr. Peiris noted that he and others had on many occasions and in print pointed out that such an event had a low probability, although it would have a high impact on human health. However, once the message passed beyond the virology community, the part about it being a low-probability event did not get through.

Hundreds of thousands of humans have probably been exposed to the H5N1 virus, Yet, only very rarely does it infect humans. “We still don't fully understand why these few people get infected and why it doesn't transmit efficiently.” But the very fact that the virus has not been able to do so suggested that it needed to undergo a complex series of changes, not just a single mutation, in order to adapt to humans, he added.

Keywords: WHOflu virusH1N1