WEDNESDAY, May 6, 2009 (HealthDay News) -- Many experts are predicting that the current outbreak of swine flu, much like the regular seasonal flu, will subside during the summer months and reappear in the fall.
That return could come with a vengeance, or not.
"We can't actually be certain, but there likely will be a reemergence," said Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City. "We're seeing this virus at a time that is usually the end of the flu season, so you would expect, because the flu virus is hardier in cold weather, that there will probably be a return."
"If you draw parallels to previous flu pandemics, the pattern has always been a mild epidemic in the early summer or late spring, then you see a larger epidemic in the winter," said Dr. Luis Z. Ostrosky, an associate professor of medicine and epidemiology in the division of infectious diseases at the University of Texas Medical School in Houston. "If this were to follow the pattern of previous outbreaks, we would see it again in the winter."
Ostrosky was referring to 1918 (Spanish flu pandemic), 1957 (Asian flu) and 1968 (Hong Kong flu).
This pattern of an initial "herald wave" followed by a second wave is common in the flu world.
"Sometimes we will see a little spike of flu towards the end of the season with that turning up next year. Maybe that's what this is. That would be very typical," said Dr. John Treanor, a professor of medicine and of microbiology and immunology at the University of Rochester Medical Center. "My guess would be that if this virus does not end up causing significant disease in the northern hemisphere over the summer, it will certainly do so in the fall."
So far, some of the most affected nations have been in North America and Europe, but the flu is spread more easily in the winter, and it's already fall in the Southern hemisphere. Experts fear public health systems could be overwhelmed if swine flu and regular flu collide in major urban populations, according to the Associated Press.
"You have this risk of an additional virus that could essentially cause two outbreaks at once," Dr. Jon Andrus, of the Pan American Health Organization's headquarters in Washington, D.C., told the wire service.
Two separate flu strains could also mutate into a new strain that is more contagious and dangerous. "We have a concern there might be some sort of reassortment, and that's something we'll be paying special attention to," World Health Organization spokesman Dick Thompson told AP.
In North America, the summer should slow down the spread of swine flu; neither viruses nor bacteria survive well at temperatures above 70 degrees Fahrenheit, explained C. Ed Hsu, an associate professor of public health informatics at the University of Texas School of Health Information Sciences at Houston and associate director of health informatics at the Center for Biosecurity and Public Health Preparedness at the University of Texas School of Public Health.
How and when the flu spreads is dependent on other factors as well: the fitness and efficiency of the virus itself along with its innate ability to replicate; the susceptibility of the host; and the environment, which includes not only the weather, but also human behavior (for example, groups of people confined together inside, making it easier for the virus to jump from person to person).
If a particular virus is especially robust, the weather and other environmental factors may play a lesser role.
"It may not care what the environment is like, because it doesn't need that assistance, or it may depend on environmental factors. It could go either way," Treanor said.
And while influenza virus needs a warm human body to replicate, it seems to sustain itself better in airborne respiratory droplets when it's cold, Horovitz said. Hence, the sneeze heard around the world.
"But, if it does go away, it will come back, he added. "I would be pretty sure of that."
A fall/winter resurgence may or may not be more virulent, said Ostrosky.
"We've been very lucky so far that it's appearing to be mild, at least in the U.S., as far as virulence and susceptibility to antivirals is concerned," he said. "It could change. That's one of the concerns."
"The severity depends on whether [and how] the genotype of the virus reassorts itself," Horovitz added. The reassortment may be so minimal as to make no clinical difference, or it could assert itself in entirely new ways."
But a vaccine will likely be ready by the time a second wave hits, Ostrosky noted, and the world is prepared in other ways as well.
"We have completely sequenced the genome of the virus, and it shows low virulence at this point. We know about it. We can prepare," he said. "If nothing else, this has been an extraordinary exercise in preparation."
The U.S. Centers for Disease Control and Prevention has more on the current outbreak of swine flu.