Many questions surround the current outbreak of swine flu, which caused the United States government on Sunday to declare a public health emergency after confirming 20 cases in this country. American officials stressed that the cases here were all mild, with only one person hospitalized.
But in Mexico, where the outbreak began, swine flu is believed to have killed at least 81 people and to have given about 1,300 serious breathing problems. The central question every flu expert in the world would like answered, Dr. Martin Cetron, director of global migration and quarantine for the Centers for Disease Control, said in an interview, is how many mild cases Mexico has had.
“We may just be looking at the tip of the iceberg, which would give you a skewed initial estimate of the case fatality rate,” he said, meaning that there may have been tens of thousands of mild infections, which would then make the number of deaths seemingly low. In that case, as the flu spreads, it would not be surprising if the number of people who become seriously ill remained relatively small.
Right now, the answer is unknowable. Only two laboratories the world, the C.D.C. in Atlanta and the Canadian national laboratory in Winnipeg, have developed the reagents needed to do a positive test for the new flu strain, so samples from any other country must be sent to them for confirmation.
Even in 1918, experts point out, the pandemic Spanish flu infected at least 500 million of the world’s 1.5 billion people, killing 50 million. Many of those lives would have been saved if anti-flu drugs, antibiotics and mechanical ventilators had existed.
In early April, Mexico had noticed that it had high numbers of serious respiratory illnesses and some deaths. It began sending samples to Canada and the United States, asking for help genotyping the new virus.
The ages of the victims in Mexico also concern health officials. Unlike typical flu seasons, when infants and the aged are usually the most vulnerable, none of the initial deaths in Mexico were in people older than 60 or younger than 3 years old, a spokeswoman with the World Health Organization said. Pandemic flus — like the 1918 flu and outbreaks in 1957 and 1968 — often strike young, healthy people the hardest. When a new virus emerges, deaths may occur in healthy adults who mount the strongest immune reactions. Their own defenses — inflammation and leaking fluid in lung cells — can essentially drown them from inside.
A second hypothesis, Dr. Cetron said, is that there is some other factor in Mexico that increased the lethality of the virus, such as co-infection with another microbe or an unwittingly dangerous form of treatment.
For example, a co-infection with the AIDS virus makes it much more likely that someone with tuberculosis will die of it. And Reyes syndrome emerged when infants with fevers were treated with aspirin.
However, Dr. Cetron emphasized that there is no evidence of any co-infection in Mexico or anything unusual about flu treatment there.
Flu experts would also like to know whether this year’s flu shots give any cross-protection against the new swine flu strains. There is an H1N1 human strain in this year’s flu shot, and all H1N1 flus are descendants of the 1918 pandemic strain. But flus pick up many mutations over the years, especially when they move back and forth between humans, pigs, birds and other hosts.
There will be no evidence for several days as to whether the shots are protective, until the C.D.C. can get stored blood samples with antibodies to the flu shot and test them against the new virus. Those tests are under way now, according to a flu expert who sent the C.D.C. his blood samples.
In several countries, students reported they were ill upon returning from spring break in Mexico. For example, three teachers and 22 students at Rangitoto College, a New Zealand high school near Auckland, were reported to be ill after arriving after a three-week trip to Mexico on a flight via Los Angeles.
Mexico’s annual flu season was trailing off in March, when there was a new spike in cases. That usually happens when B strain flus peak late in the season, but B strains are usually mild. Mexico sent samples to the United States and Canada after its officials realized the country was having an unusual outbreak with severe cases.
