Research Stories

by Diane Boudreau

The Oakland (Calif.) Municipal Auditorium in use as a temporary hospital during the 1918 flu epidemic. Photo by Edward A. "Doc" Rogers, courtesy of the Oakland Public Library.

Swine flu is a little bit like the kid sister who lives in the shadow of her older sibling. When people talk about the H1N1 flu that broke out in Mexico in April of this year, they invariably hearken back to the last H1N1 pandemic. That would be the Spanish flu of 1918. That one is thought to have killed 20 million to 50 million people worldwide.

The kid sister isn’t living up to the family legacy so far, which is good news. But the family resemblance is uncanny. Influenza pandemics share certain characteristics that set them apart from regular seasonal flu. Scientists and public officials are studying the features of past pandemics in order to deal with today’s virus and potential future outbreaks.

Gerardo Chowell-Puente is a mathematical epidemiologist in ASU’s School of Human Evolution and Social Change. He explains the distinctive features of pandemic flu and why we still need to be cautious about H1N1:

Pandemic flu is the new kid in town
“Past pandemics occurred because a new flu strain emerged,” explains Chowell-Puente, who studies the transmission of infectious disease—particularly influenza. “Typically, that new strain replaces or dominates prior strains. The new strain persists in the population and just changes slightly to ensure its survival over epidemic seasons.”

The current pandemic is known more formally as swine-origin influenza A (H1N1). The “A” refers to the influenza type. There are three types of flu: A, B and C. All influenza pandemics and currently dominant flu strains are due to influenza type A.

Influenza A viruses are broken down into subtypes based on two proteins found on the surface of the virus. Those proteins are labeled H and N. There are 16 H and nine N subtypes known. The dominant flu subtype since it emerged in 1968 has been H3N2.

Within a subtype, different strains of virus can emerge. The 2009 swine flu is a new strain of H1N1 influenza. It contains genetic components from human, swine, and avian influenza viruses.

Pandemic flu arrives early and usually makes a comeback
The flu season typically starts in October and ends in March at the latest. Pandemic flu, in contrast, can show up unexpectedly in spring or summer. The most worrisome aspect of pandemics, however, is that they tend to appear in a series of “waves.”

“The 1918 pandemic came in a series of waves. The first arrived in the spring/summer. What we see right now with swine flu looks consistent with the summer wave of the 1918 pandemic,” says Chowell-Puente.

American Red Cross volunteers of 1917-1918 preparing surgical dressings. Photo by Edward A. "Doc" Rogers, courtesy of the Oakland Public Library.

He notes that the first wave of the 1918 pandemic looked a lot like seasonal flu in terms of transmission and case fatality rates. This swine flu outbreak also looks a lot like seasonal flu so far, which is why the World Health Organization has declared it only a “moderate” pandemic at this point.

However, the 1918 flu struck in two to three separate waves. The second wave began during the fall. It was much more virulent than the first.

No one knows if swine flu will return with a vengeance this fall, but health officials are preparing for the possibility. Pharmaceutical companies are working on a novel vaccine for H1N1 and building up antiviral supplies. Meanwhile, scientists and public health officials are working together to model likely scenarios for a return and the best strategies for dealing with it. (See story, “Tough choices about flu”)

From June 25-28, ASU hosted a conference for researchers and public health officials from the U.S., Mexico and Canada called “Mitigating the spread of A/H1N1: Lessons learned from past outbreaks.” The event was organized quickly after the initial outbreak by Carlos Castillo-Chavez, director of ASU’s Mathematical, Computational and Modeling Sciences Center, with help from graduate students in the program. Chowell-Puente also presented at the conference.

Pandemic flu targets the young
Perhaps the most distinctive feature of pandemic flu is that it tends to affect the younger population with higher severity. This contrasts sharply with seasonal flu, which is most dangerous to infants and elderly people.

“The age shift is one of the features that raise the alarm about flu,” says Chowell-Puente. The 1918 pandemic had high mortality rates among young adults. The swine flu is following a similar pattern.

The vast majority of H1N1-related deaths and cases of severe pneumonia in Mexico happened to people between ages 5 to 59, according to a study co-authored by Chowell-Puente. The study appeared June 29 online in the New England Journal of Medicine. Another study in Japan showed that nearly 80 percent of H1N1 cases during the early pandemic phase affected people aged 10 to 19. This study, co-authored by Chowell-Puente and Castillo-Chavez along with colleagues in Egypt and the Netherlands, appeared in the June 4 issue of Eurosurveillance.

This sort of information is crucial for devising effective strategies to combat the flu.

“Because achieving high vaccination rates before the fall is not feasible with current technologies, effective distribution of a limited vaccine and antiviral stockpiles will be crucial to mitigate a potential second pandemic wave,” says Chowell-Puente. “The seasonal influenza vaccination strategy focuses on the very young and the very old—the most vulnerable populations. This is not necessarily the case for pandemics, as we showed in our study.”

Moving forward
Humanity breathed a collective sigh of relief when we learned that the new H1N1 strain is no more threatening than the typical seasonal flu. But we may not be saying goodbye to swine flu just yet. Unlike past pandemics, however, we now have vaccines, antiviral drugs, and antibiotics to treat secondary infections that often result from the flu. We have supercomputers that help us model strategies for mitigation. We have rapid global communication systems. And we have the lessons of history to help guide us into the future.

Read more about how mathematical models are helping health officials make sound decisions in "Tough choices about flu"

Read more about Chowell-Puente’s NEJM study

Read more about the flu conference at ASU