Oct. 29 (Bloomberg) -- Swine flu may have infected as many as 5.7 million people in an initial wave that swept across the U.S. earlier this year, researchers at the Centers for Disease Control and Prevention and Harvard School of Public Health said. The number of swine flu patients in the U.S. in spring may have been up to 140 times greater than the reported number of confirmed cases, according to a study published in the CDC journal Emerging Infectious Diseases yesterday. A model used by the researchers to extrapolate total cases suggests 1.8 million to 5.7 million infections occurred from April to July.
More than 414,945 people worldwide have caught the new H1N1 strain since the first influenza pandemic virus in 41 years was discovered in Mexico and the U.S. in April, according to World Health Organization data. The tally is “significantly lower than the actual number of cases that have occurred,” the Geneva-based WHO says, because many countries have stopped counting individual cases, particularly of milder illness.
“Relying on laboratory-confirmed cases limits the ability to understand the full impact and severity of the epidemic, especially when severe cases are more likely to be recognized,” Carrie Reed and colleagues at the CDC and Harvard’s Marc Lipsitch wrote. “Health systems and infrastructure may be unprepared in the short-term if plans are based on a number of confirmed cases.”
The fast-moving pandemic spread to 177 countries in four months, yet causes little more than a fever and a cough in the majority of cases. Flu activity is now widespread in 46 U.S., states in a second, fall wave, the CDC said on Oct. 23.
Visits to doctors for influenza-like illness are “increasing steeply and are now higher than what is seen at the peak of many regular flu seasons,” the agency in Atlanta said.
14,000 Hospitalizations
Using their so-called multiplier model, Reed and colleagues estimated that every reported case of pandemic H1N1 flu may represent 79 total cases, for a median estimate of 3 million symptomatic cases and 14,000 hospitalizations in the U.S.
They calculated that the total number of hospitalized H1N1 patients may be about 2.7 times higher than reported. While they didn’t use the model to estimate mortality during the first four months of the pandemic, the researchers said that if deaths and hospitalizations are underreported to the same extent, about 800 fatal cases might have occurred. That compares with 302 laboratory-confirmed fatalities nationwide through July 23.
“This model provides a relatively quick and simple approach to estimate the human health impact of the epidemic in advance of more rigorous analysis of surveillance and health care data that will be available over the next few years,” the authors wrote.
They estimated that the incidence of infection over the first four months of the pandemic in the U.S. ranged from 107 cases per 100,000 people 65 years and older to 2,196 cases per 100,000 people aged 5 to 24 years.
“In some heavily affected areas, the size of the outbreak quickly exceeded the capacity to ascertain and test case- patients,” the authors said. “Thus, our results may reflect a conservative estimate of total cases.”