H1N1 swine flu virus highly infectious

Secondary attack rate of H1N1 ranges from 22% to 33%, says WHO

The Swine flu-causing H1N1 influenza A virus is so deadly and infectious that it is capable of leading to a global epidemic outbreak similar in magnitude to the earlier one in 1957 that killed over 2 billion people.

The epidemic of influenza A (H1N1), which is thought to have started in Mexico on 15 February 2009, has infected around 23,000 people by the end of April, and killed 91 people, according to the early findings about the emerging pandemic published in the journal Science.

Researchers say these data so far is very consistent with what they would expect to find in the early stages of a pandemic.

“Our study shows that this virus is spreading just as we would expect for the early stages of a flu pandemic. So far, it has been following a very similar pattern to the flu pandemic in 1957, in terms of the proportion of people who are becoming infected and the percentage of potentially fatal cases that we are seeing,” said professor Neil Ferguson, the corresponding author of the new research from the MRC Centre for Outbreak Analysis and Modelling at Imperial College London.

Researchers are working in collaboration with the World Health Organisation and public health agencies in Mexico.

“What we’re seeing is not the same as seasonal flu and there is still cause for concern – we would expect this pandemic to at least double the burden on our healthcare systems. However, this initial modelling suggests that the H1N1 virus is not as easily transmitted or as lethal as that found in the flu pandemic in 1918,” added Professor Ferguson.

However, the figures are uncertain – for example, some mild cases may have gone unreported. The numbers infected could be as low as 6,000 people or as high as 32,000 people.

The uncertainty around the numbers of people who have been infected with influenza A (H1N1) in Mexico means that the case fatality ratio (CFR) of 0.4% (4 deaths per 1000) cannot be definitely established. The CFR is in the range of 0.3% to 1.5%, but at this stage the researchers believe that 0.4% is the most likely.

Researchers suggest that for every person infected, it is likely that there will be between 1.2 and 1.6 secondary cases. This is high compared to normal seasonal influenza, where around 10-15 percent of the population are likely to become infected. However, it is lower than would be expected for pandemic influenza, where 20-30 percent of the population are likely to become infected.

The influenza A (H1N1) is found fatal in around 4 in 1,000 cases in Mexico, which would make this strain of influenza as lethal as the one found in the 1957 pandemic.

The data also suggest that the H1N1 strain infects youngsters more than adults. In Mexican village called La Gloria, children were twice as likely to become infected as adults, with 61% of those aged under 15 becoming infected, compared with 29% of those over 15.

Researchers see this indicating that adults have some degree of immunity against infection, because of having been previously infected with a related strain of influenza, or it may mean that children are more susceptible to infection because they interact much more closely together, for example in school, than adults.

The H1N1 virus strain causing the current outbreaks is a new virus that has not been seen previously in either humans or animals. Although firm conclusions cannot be reached at present, scientists anticipate that pre-existing immunity to the virus will be low or non-existent, or largely confined to older population groups.

H1N1 appears to be more contagious than seasonal influenza. The secondary attack rate of seasonal influenza ranges from 5% to 15%. Current estimates of the secondary attack rate of H1N1 range from 22% to 33%, according to a statement on the WHO’s Web site.

Meanwhile, ten new cases of swine flu have been confirmed in England, taking the total number in the UK to 65.

Swine flu has been confirmed in 4,694 people, according to WHO. Sixty-one people have died, including 56 in Mexico, three in the U.S., and one each in Canada and Costa Rica. The U.S. confirmed 2,618 cases in 44 states, according to the Centers for Disease Control and Prevention.