Early flu wave may protect against worse one later

September - October, 2008  |  Volume 7, Issue 5

a black and white photo of Seattle police wearing flu masks standing shoulder to shoulder, during the 1918-1919 pandemic.
Seattle police wearing masks to prevent catching the flu during
the 1918-1919 pandemic. Photo: National Archives

New evidence about the worldwide influenza pandemic of 1918-1919 indicates that getting the flu early protected many people against a second deadlier wave, an article co-authored by a Fogarty epidemiologist concludes.

American soldiers, British sailors and a group of British civilians who were afflicted by the first mild wave of influenza in early 1918 apparently were more immune than others to the severe clinical effects of a more virulent strain later in the year, according to the paper in the Nov. 15 issue of the Journal of Infectious Diseases by medical historian John Barry, staff scientist Cecile Viboud, Ph.D. and epidemiologist Lone Simonson, Ph.D., of The George Washington University.

"If a mild wave is documented, the benefits of cross-protection during future waves should be considered before implementing public health interventions designed to limit exposure," the authors suggested.

Mark Miller, M.D., director of the Fogarty Center's Division of International Epidemiology and Population Studies, said the finding could have implications for future pandemics. "If a 1918-like pandemic were to repeat itself, the early circulation of less pathogenic pandemic viruses could provide some level of population immunity that would limit the full onslaught from the second wave."

"Together with historical data recently uncovered from Denmark and New York City, this study gives us a different look at the process of adaption of novel pandemic influenza viruses to humans and the evolution of virulence," Viboud said.

Among soldiers at U.S. training camps, the rate of illness among soldiers was 3.4 times higher among those who had not previously had the flu, and the rate of death per case was about five times as high.

For people who were infected in the first wave, the risk of illnesses in the second wave was reduced by between 35 percent to 94 percent, about the same protection as for modern vaccines--70 percent to 90 percent. The risk of death was reduced between 56 percent to 89 percent.

At one base, a regiment that had transferred in from Hawaii where soldiers who were exposed to the spring wave had a 6.6 percent incidence in the fall compared to 48.5 percent in a regiment transferring in from Alaska, where soldiers had not been exposed.

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