Swine flu wanes, but experts say pandemic strain could reemerge
Tuesday, February 23, 2010
Even as officials from the Centers for Disease Control and Prevention are announcing that the epidemic of the H1N1 flu is no longer widespread in any state, no disease expert is willing to say there isn't a third -- or fourth -- wave of swine flu in the country's future.
Influenza transmission waxes and wanes, and outbreaks of novel pandemic strains occur in particularly unpredictable waves that depend on such variables as human behavior, atmospheric conditions and even competition from other microbes. That places them among the bigger mysteries of epidemiology, the science of disease outbreaks.
The "Spanish flu" of 1918 had four waves of greatly differing deadliness, spread over two years. The "Asian flu" of 1957, like the current H1N1 strain, had a late-spring and a fall wave -- followed by a third in late winter of 1958. It then took a year off before peaking again in 1960. The "Hong Kong flu" of 1968 had more than a year hiatus between its two waves, with the second infecting nearly as many people as the first.
"We are not at all out of the woods because the virus continues to circulate, but the chances of a very large additional wave are very hard to predict," said Anne M. Schuchat, who is leading the government's response to the H1N1 pandemic at the CDC.
What explains the retreat of this flu since its peak in late October? "That's a great question, and it is still very much an open question," said Katia Koelle, a biologist at Duke University who studies the dynamics of disease outbreaks.
Why doesn't a flu outbreak last all winter, when the optimal conditions of cool temperatures, low humidity and crowded living are present? "It just doesn't. It runs through a community and moves on," said Walter R. Dowdle, who worked at CDC during the 1968 pandemic and is now an epidemiologist at the Task Force for Global Health in Decatur, Ga.
"Why do we have waves? I can't find anybody who can tell me a biological explanation that makes sense," said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
At one level, the reason for H1N1's waning is straightforward. People infected with the virus are passing it on to fewer people now than they were last fall.
The growth and maintenance of a disease outbreak is a matter of simple arithmetic. If each person carrying a microbe passes it on to more than one person, the epidemic will spread. If each person passes it, on average, to less than one person -- which means that many people don't transmit it to anyone -- then the epidemic will eventually burn out.
Let's do the numbers
How many people the average infected person infects is called the basic reproductive number, or R0 (pronounced "R naught"). It is a crucial feature of each microbe's personality. Some microbes are simply more transmissible than others; they have a higher R0.
Measles, which is probably mankind's most contagious infection, has an R0 of about 18. Polio's number is about 6; severe acute respiratory syndrome (SARS) about 5. For seasonal flu strains, the R0 is about 1.2, and for pandemic strains it is rarely higher than 2. For the novel H1N1 strain (also known as swine flu), it's about 1.6.
What this low R0 means is that flu outbreaks are always teetering on the verge of having their myriad chains of transmission broken by people who get infected but don't pass the virus to anyone else.