TUESDAY, Aug. 11, 2009 (HealthDay News) -- The theory that a relatively mild outbreak of a new flu virus in the spring predicts a more severe, deadly outbreak in the fall isn't borne out by a look back at prior epidemics, two U.S. experts say.
"Pandemic history suggests that changes neither in transmissibility nor in pathogenicity are inevitable," concluded Drs. David Morens and Jeffery Taubenberger, infectious disease experts at the U.S. National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.
In an article published in the Aug. 12 issue of the Journal of the American Medical Association, the experts take on a much-publicized theory that's helped stoke fears about a resurgence of swine flu in the Northern Hemisphere this fall.
The so-called "herald wave" theory stems from the belief that the deadly 1918-19 flu pandemic began with a milder spring wave of illness, which got more deadly as the virus spread throughout the summer, picking up lethal mutations. The 1918-19 "Spanish Flu" is estimated to have killed between 20 million and 40 million people worldwide.
However, while flu outbreaks were noted in Europe in the spring of 1918, no viruses from these outbreaks "have yet been identified," Morens and Taubenberger noted. And the actual course of the 1918 pandemic flu varied greatly around the world -- most areas experienced no "spring wave" at all, and the timing of successive waves changed between regions and even between countries, the researchers said.
The two experts also broadened their review to look at 14 major flu epidemics that have swept the globe since the beginning of the 16th century.
"In doing so, it is difficult to find evidence of 1918-like waves herald waves, or other such phenomena," they noted. The most recent flu pandemics, occurring in 1957 and 1968, "generally exhibited no more than one (mostly seasonal) recurrence" before settling down into relatively innocuous seasonal flu, they said.
Overall, "examination of past pandemics reveals a great diversity of severity," Morens and Taubenberger said, adding that "some newer evidence [is] casting doubt on original herald wave theories."
One infectious-disease expert called the new analysis "absolutely correct."
Looking back at 20th century flu pandemics, "secondary waves have pretty much been either the same or even of less epidemiologic significance than the first wave," said Dr. Pascal James Imperato, dean of the school of public health at SUNY Downstate Medical Center in New York City.
And as for the current H1N1 swine flu pandemic, the NIAID experts believe that the relatively poor transmissibility of the virus, the fact that many people have some pre-existing immunity, and its arrival in the Northern Hemisphere in late spring "all give reason to hope for a more indolent pandemic course and fewer deaths than in many past pandemics."
Imperato concurred with that assessment. Swine flu is "still circulating," he said, "and that means that a lot of people have developed protection against it, plus we have the advantage that it's a descendant of other H1N1 viruses that were in circulation in the late '70s through the '80s, so older people have solid protection."
"It's hard to conceive that if the H1N1 should reappear in the fall in the Northern Hemisphere that we would have a more severe epidemic," he said.
This should come as good news as the United States gears up for the coming fall flu season. This week, volunteers began lining up at centers nationwide as the first swine flu vaccine trials began.
"The best way to prevent the spread of the flu is vaccination," U.S. Department of Health and Human Services Secretary Kathleen Sebelius told reporters on Friday, "and our scientists are working hard to have a vaccine ready for consumption by mid-October."
Over 120 million doses are expected to be delivered, with priority going to health-care workers, pregnant women and people with underlying health conditions, federal officials have said.
Those efforts should be supported, Imperato said, even if the fall flu season turns out to be relatively benign.
"I think it's prudent to do what is being done now. You prepare for the worst but hope for the best," he said.
Find out the latest on H1N1 swine flu at the U.S. Centers for Disease Control and Prevention.