World health officials have given up trying to count how many people have the H1N1 swine flu virus, saying the best they can do is estimate the spread of the "unstoppable" virus.
But yesterday, Anne Schuchat of the US Centers for Disease Control and Prevention declined even to hazard a guess, saying it was "more than a million."
Why not? Because officials do not have the tools they need to count.
"Most people who have respiratory illnesses don't find out exactly what caused it. Even most people with influenza don't know exactly which type of influenza caused their illness," Schuchat told reporters.
Diagnostic tests for influenza are not very good. In fact, swine flu is often suspected when a patient's instant in-office flu test comes up unreadable.
"I think that we all know the diagnostics we have, have limitations," Robin Robinson, of the US Department of Health and Human Services, told flu vaccine experts meeting this week.
"I think we would all agree that there is a need to have improved point-of-care diagnostic testing," said pediatrician John Modlin, who chairs the vaccine committee that advises the Food and Drug Administration.
Aware of the need, the FDA on Friday authorized emergency use of a Quest Diagnostics Inc test for the H1N1 swine flu virus, only the third commercial test available.
Earlier this month, the World Health Organisation said it would stop issuing counts of confirmed cases of swine flu, saying it was more important to concentrate efforts elsewhere.
Yesterday, the CDC said it would drop its weekly swine flu count too.
"We think that millions and millions of people are affected," Schuchat said -- but added that was just the tip of the iceberg because only the most serious cases usually make it to the hospital to be tested.
"We're no longer going to expect the states will continue this individual reporting and we're going to transition to other ways of describing the illness and the pattern," Schuchat said.
ATTACK RATES
What would those be? They are so-called models, in which health experts take case studies and extrapolate from them how many people get flu.
For instance, if one person is diagnosed with H1N1, confirmed by a lab test, health investigators from the CDC or state health departments can then carefully watch family members to see who else gets infected. They can also see how many co-workers and other contacts become infected.
They can also watch entire communities and estimate the "attack rate."
H1N1, for instance, appears to have infected between 6 percent and 8 percent of people in communities where it was really active.
"But this virus didn't circulate everywhere this past spring," Schuchat said. "To some some extent, we were seeing a lot of transmission when the circumstances weren't that great for transmission," she added.
"So we think in a longer winter season, attack rates would probably reach higher levels than that, that we would see quite a bit more than that -- maybe more two or three times as high as that."
Each year, seasonal flu infects about 20 million people in the United States alone, CDC estimates. But that is a virus many people have had several times, so there is a high level of general immunity.
H1N1 is new to just about everybody's immune system, so more people may be susceptible.
Death rates are still unclear, too. Each year, influenza is involved in anywhere between 250,000 and 500,000 deaths globally, the WHO estimates -- based on some of those kinds of models.
The WHO has reported 800 confirmed deaths globally from the H1N1 virus, which has spread to 160 countries. But health experts point out that figure does not reflect the true number of people killed by the virus. Flu can kill people in any number of ways, by causing pneumonia, heart attack, stroke and multiple organ failure.