If a flu pandemic similar to the deadly one that spread in 1918 occurs, it may be possible to keep the pandemic in check through vaccinations, a new study suggests. The infamous 1918 pandemic killed up to 40 million people worldwide, but the virus strain was not unusually contagious compared with other infectious diseases such as measles, according to a new analysis by researchers at the Harvard School of Public Health (HSPH). However, the 1918 flu was quite lethal once contracted, believed to be 10 times more lethal than other pandemic strains.
Epidemiologists analyzed historical epidemic data in 45 U.S. cities and found that the transmissibility of the 1918 strain as measured by the number of people infected by a single case was only about 2 to 4, making the strain about as transmissible as the recent SARS coronavirus. Because people with influenza can transmit the infection before the appearance of symptoms, strategies for transmission reduction, including vaccination, would need to be implemented more rapidly than measures for the control of the recent SARS outbreak. Isolation methods alone, such as those used for controlling SARS, would not be effective.
The analysis, “Transmissibility of 1918 Pandemic Influenza,” by Christina Mills, doctoral student in the HSPH Department of Epidemiology, and faculty co-authors appears in the Dec. 16 issue of Nature.
World Health Organization officials have warned that we are closer now to another pandemic than in any other recent time because of the persistent bird flu epidemic in East Asia that threatens to jump to humans.
“Our study suggests that if you could vaccinate a reasonable number of people before exposure, a future flu epidemic could be controlled,” said Mills. “But flu takes just a couple of days to become contagious in a person, unlike the almost weeklong latency period we experienced with SARS. Isolation would be only partially effective. Rapid vaccination is essential.”
“Before this study, estimates were all over the map on the transmissibility of pandemic flu,” said co-author Marc Lipsitch, associate professor of epidemiology at HSPH. “Some thought it was so transmissible that vaccines would be unlikely to stop it. This study is optimistic, except we don’t have the vaccine. It is now even more important to put resources into the development of vaccine technology, manufacture and distribution systems to make possible a rapid response to the next outbreak of an entirely new flu strain.”
“This year we found out how bad our system is currently with the contamination of flu vaccine at one manufacturer and the consequent shortage,” said Lipsitch. “We are lucky this is happening in a year when the flu is not so terrible. We need to have our manufacturing system functional quickly.”