Swine flu sweeps the southern hemisphere

• 09 July 2009 by Debora MacKenzie

THE swine flu pandemic is intensifying. The White House will meet with state representatives on 9 July to discuss preparations for the autumn flu season in the US, while the UK has shifted its response to dealing with widespread infection.

Meanwhile, in the southern hemisphere, in the midst of its winter flu season, swine H1N1 virus seems to be replacing the seasonal flu viruses that circulated till now - classic pandemic behaviour. This raises concerns that seasonal flu vaccine, which some companies are still making, may be useless when the northern hemisphere's flu season arrives later this year.

In the flu pandemics of 1918, 1957 and 1968, the pandemic virus completely replaced the circulating seasonal flu. But in 1977, an accidentally released mild H1N1 virus simply circulated alongside the existing flu, H3N2. So no one is sure how swine H1N1 will behave. If it does not replace the seasonal viruses - the milder H1N1 and the H3N2 - the world faces the prospect of all three viruses at once. This would be a complicated scenario: both seasonal and pandemic vaccines would be needed and differing age groups of people would be affected. From what is happening in the southern hemisphere, though, it appears that may not happen.

In the northern hemisphere, swine flu also dominates: more than 98 per cent of flu cases genotyped in the US in late June were caused by the pandemic virus. This is to be expected. While seasonal flu viruses normally die out in the summer, the pandemic virus has the advantage that few people have any immunity to it.

In Chile 98 per cent of flu cases are caused by swine H1N1. The seasonal vaccine is useless

In Australia, the state of Victoria, the hardest hit so far, reported this week that swine H1N1 now accounts for 99 per cent of all flu cases. It is a similar story in South America. In Chile, swine H1N1 is also outrunning the seasonal virus. "Ninety-eight per cent of the flu cases we have now are caused by H1N1," Chile's under-secretary of public health, Jeanette Vega, told a pandemic summit in Cancún, Mexico, last week. "The seasonal vaccine is useless."

In Argentina, where authorities in the capital Buenos Aires declared a health emergency last week, health minister Juan Manzur says 90 per cent of the flu virus in circulation is swine H1N1.

For the coming winter in the northern hemisphere, this is a concern. "If the pandemic virus massively outcompetes the seasonal viruses in a regular flu season, the seasonal viruses are likely to be replaced by the new virus, like in the 1968 pandemic," says Ab Osterhaus of the University of Rotterdam in the Netherlands.

In previous pandemics, the virus has then mutated, making its effects worse. So far H1N1 has acquired no obvious new mutations, but a few ominous signs have emerged.

A mutation to the virus's polymerase enzyme, which makes it replicate more efficiently, has cropped up in a sample from Shanghai, China. This could spread if it makes the virus more contagious. But it may also increase pathogenicity, says Ron Fouchier of the University of Rotterdam.

And last week, two cases of swine H1N1 with resistance to the main antiviral drug, Tamiflu, were discovered in people using the drug. Another was discovered in a girl who had never taken the drug, suggesting Tamiflu-resistant swine H1N1 might already be circulating.