Early Pandemic Flu Wave May Protect Against Worse One Later

Evidence Shows Spring Outbreak in 1918 May Have Immunized Against Deadlier Second Wave

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 an NIH 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 published in the Nov. 15 issue of the Journal of Infectious Diseasesby medical historian John Barry, staff scientist Cécile Viboud, Ph.D., of the NIH’s Fogarty International Center and epidemiologist Lone Simonson, Ph.D., of The George Washington University.

"If a mild first 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 adaptation of novel pandemic influenza viruses to humans and the evolution of virulence" Viboud said.

The researchers pored over medical data from U.S. Army bases, the British fleet and several British civilian communities, applying modern mathematical models to study the pandemic. They determined that in the spring of 1918, influenza occurred at different levels of severity throughout the United States, and was not always recognized as a pandemic. By the fall, however, 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.

The disparity was not as great for the British sailors and civilians whose records were studied.

For people who were infected in the first wave, the risk of illness 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.

The authors found that while there were variations in overall influenza cases among the 37 U.S. Army bases in the spring of 1918, soldiers who had been sick in the spring experienced lower rates of illness and death during the more lethal pandemic outbreak in the fall. At one base, a regiment that had transferred in from Hawaii where soldiers 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.

The study suggests two possible reasons for the difference in incidence and lethality between the first two waves: a relatively weak virus mutating into a stronger one or a respiratory bug in the fall making flu patients sicker.

The 1918-1919 pandemic killed between 50 million and 100 million people worldwide and was unusually deadly in young adults, including soldiers.

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