At least one million people in the United States have had swine flu, or around 50 times more than the number of cases reported to health authorities, US health authorities said Friday.
"We're saying that there have been at least a million cases of the new H1N1 virus so far this year in the United States," said Anne Schuchat of the Centers for Disease Control and Prevention (CDC).
"Reported cases are really just the tip of the iceberg," said Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, of the roughly 28,000 confirmed cases of A(H1N1) flu in the United States.
Around 3,000 people infected with swine flu in the United States have had to be hospitalized and 127 people are reported to have died.
The CDC arrived at its figure of one million cases based on computer models and surveys of communities known to have been hard hit by the new flu strain.
"There have been community surveys in a couple of areas looking at influenza-like illness in areas where we know there's a lot of the strain circulating and in many of those communities, they're reporting proportions of about 6 percent of community members having had an illness that's consistent with the new virus," Schuchat told reporters.
A community survey conducted in New York City, where the CDC believes there have been half a million cases of A(H1N1), showed 6.9 percent of residents experienced flu-like illness during a three-week period in May, Schuchat said.
"From their virologic testing, they knew that most of that influenza-like illness was based on this new H1N1 strain, and from that, they estimated that around half a million New York City residents may have been infected with this new virus and had flu-like illness without necessarily seeking care," Schuchat said.
Although an infection rate of around six percent is low compared with seasonal influenza, said the CDC official, "the survey in New York looked at a several-week attack rate while seasonal flu usually happens over weeks to months."
Schuchat said the actual figure of infected people in the United States was probably higher than one million and warned that swine flu might see higher infection rates than seasonal influenza.
"We believe the attack rates of this new virus, particularly in young people, may exceed the kind of attack rates we see with seasonal influenza," she said.
The highest rates of illness due to A(H1N1) flu are in people under the age of 25. The median age of people who have been hospitalized in the United States after being infected with the virus is 19, and the median age of those who have died is 37.
Schuchat repeated a warning that the new strain of swine flu could come back in a more virulent form with the return of flu season in the autumn and urged communities to begin mapping out a vaccination campaign.
Five companies are working to develop a vaccine against A(H1N1) flu, said Schuchat, without naming them.