By John Lauerman
Last Updated: May 12, 2009 02:24 EDT

May 12 (Bloomberg) — The swine flu strain that has sickened people in 30 countries rivals the severity of the 1957 “Asian flu” pandemic that killed 2 million people, scientists said.

About four of 1,000 people infected with the new H1N1 strain in Mexico by late April died, according to a study published yesterday in the journal Science that was led by Neil Ferguson of the Imperial College London. Seasonal flu epidemics cause 250,000 to 500,000 deaths each year, the World Health Organization has said.

Scientists are trying to determine whether swine flu will mutate and become more deadly as it spreads to the Southern Hemisphere and back. The virus is more contagious than seasonal flu, the Geneva-based WHO said yesterday. A “moderate” pandemic like the 1957 Asian flu could kill 14.2 million people and shave 2 percent from the global economy in the first year, the World Bank said in October.

“While substantial uncertainty remains, clinical severity appears less than that seen in 1918 but comparable with that seen in 1957,” the Science study authors wrote.

Flu pandemics occur when a strain of the disease to which few people have immunity evolves and begins spreading. Pandemics usually occur two to three times a century, scientists have said. A worldwide outbreak as severe as the 1918 Spanish flu might cause 180 million to 360 million deaths, according to a 2005 study in the New England Journal of Medicine.

The last pandemic hit in 1968, and health officials have been anticipating another since the H5N1 strain began spreading widely in birds in 2003.


Each person infected with swine flu in Mexico in April gave it to 1.4 more people on average, the study said. While that’s in the lower range of transmission speed for a pandemic virus, it’s quicker than most seasonal flus, the authors said.

An estimated 23,000 people in Mexico were infected by late April, the researchers said. That number was based on case reports and assumptions about the speed of spread, and may have been as high as 32,000 and as low as 6,000, according to the study.

More Contagious

In seasonal flu, each person who comes in contact with someone who’s sick has a 5 percent to 15 percent probability of illness, according to a statement on the WHO’s Web site. In swine flu, the probability increases to 22 percent to 33 percent, WHO said.

Swine flu has been “overwhelmingly mild outside Mexico,” the WHO statement said. The reason for that variation “is still not fully understood,” it said.

Swine flu is making more young people seriously ill, compared with seasonal flu, and “is of particular concern” because it’s causing more significant medical effects in people with other health conditions, the WHO said.

To contact the reporter on this story: John Lauerman in Boston at