As many as 3 million Americans may now be immune to the West Nile virus thanks to antibodies they produced after being infected by the bite of an infected mosquito.
And a tenth of 1 percent of the population - about 300,000 people - acquire new West Nile infections each year, most without ever experiencing any symptoms of the disease, according to a study in the current issue of the journal Emerging Infectious Diseases.
"We do not see any indication that that trend will not continue," said Thomas R. Kriel, senior director of viral vaccines at Baxter International Inc., in Vienna, Austria. Kriel is the senior author of the study in the journal, which is published by the U.S. Centers for Disease Control and Prevention.
The researchers were able to draw their conclusions without asking millions of people for costly blood samples.
Instead, they tested about 600 samples of immune globulin, a blood product derived from blood and blood plasma donated to the American Red Cross. Each sample contains antibodies acquired from an average of 10,000 donors. The product is used to treat people with immune deficiencies.
"By testing a single sample of [immune globulin] an average of the West Nile virus infection history can be obtained for thousands of blood and plasma donors," Kriel said.
And by looking at 600 samples, the researchers were able to estimate the immunity levels of millions of donors - enough to be a statistically representative sample of the entire U.S. population of 300 million.
Previous "sero-surveys" looking at West Nile antibody levels in just a few thousand individuals have found that between 2 percent and 14 percent of the populations studied had enough antibodies to confer immunity, a variation Kriel attributes to geography, and to the intensity of viral activity in those locations.
But with their broader reach through immune globulin samples, Kriel and his team concluded that about 1 percent of the U.S. population - or at least that portion of the population that donates blood - now has some level of West Nile immunity, or more than 3 million people.
The West Nile virus first arrived in the New York City area in 1999. It quickly began sickening and killing birds and people, and birds subsequently spread it south and west. It eventually became endemic across all 48 contiguous states. U.S. fatalities peaked in 2003, with 264 killed by the virus.