Swine flu spread justifies treating it as a pandemic

H1N1 swine flu is spreading fast enough to justify the preparations for a pandemic, say epidemiologists who have analyzed the pattern of spread so far.

“The message is that the epidemic is spreading very much as expected based on past flu epidemics,” says Christophe Fraser of Imperial College London, and co-leader of the analysis team. The results suggest that the H1N1 virus is showing “sustained human-to-human transmission,” thereby justifying the WHO’s pandemic phase 5 rating, one short of the most severe.

New Scientist’s Andy Coghlan writes that by analyzing the pattern of spread within and beyond Mexico, where the epidemic emerged in February, the researchers estimate that on average, each person who contracts flu passes it on to between 1.4 and 1.6 other people.

Note that whenever this value, called the reproductive number or R_o, is more than 1, it means that a disease is transmissible, and the higher the number, the easier it is transmitted. The values found match or are less than those for previous flu epidemics, in 1918, 1957, and 1968.

While the speed of spread is now better understood, researchers say they need more data to assess how dangerous the new virus is. “Our early analysis would suggest this is going to be an outbreak comparable to that of 20th century pandemics regarding the extent of its spread,” says Neil Ferguson, co-leader of the team. “However, it’s very difficult to quantify the human health impact at this stage,” he said.

Their best estimate from the Mexican fatalities is that the virus kills 0.4 percent of those it infects on average, ranging from 0.3 to 1.5. “So while substantial uncertainty remains, clinical severity appears less than that seen in 1918, but comparable with that seen in 1957,” says the journal report. This, however, is the “first wave” of the virus — scientists don’t yet know whether the virus will become more dangerous.

Ferguson and his colleagues reached their latest estimates by assuming that cases in visitors to Mexico were much better investigated than those in Mexico itself, where the virus circulated invisibly for months before fuller surveillance was introduced. “We used indirect detective measures to work it out,” says co-leader of the team, Christophe Fraser, also at Imperial. “We had good data on people crossing in and out of Mexico, and assumed they were representative of the numbers of people originally infected in Mexico,” he explained.

From the figures, they estimate that between 6,000 and 32,000 individuals in Mexico have had the virus, with a central estimate of 22,000. From this, they worked out the R_o estimates.

They reached a similar R_o figure, 1.2, when they worked backward by tracking genetic mutations in the virus since the epidemic originated. The team says there is more uncertainty about how lethal the virus is, because there have been relatively few deaths so far. “That’s the tricky one,” says Fraser.

Figures released today by the WHO reveal that there have been 4,694 cases worldwide, with 48 deaths in Mexico, three in the United States, and 1 each in Canada and Costa Rica. “Much remains to estimate the clinical severity of infection,” say the researchers.

Equally difficult is to predict how the virus will evolve. They warn, though, that as the flu season gets underway in the southern hemisphere, it will need to be carefully monitored to find out whether climatic conditions make things better or worse, whether the virus develops resistance to anti-flu drugs, and whether vaccination makes any difference to survival. “The key trade-off remains the balancing of the economic and societal cost of interventions, such as school closure, against the numbers of lives saved through use of such measures,” they say. “Where substantial antiviral stockpiles are available, a secondary trade-off is the extent to which large-scale prophylaxis is justified, given the potential of high level resistance developing.”

-read more in Neil Ferguson et al., “Pandemic Potential of a Novel Strain of Influenza A (H1N1): Early Findings,” Science (11 May 2009)