Cracking the swine flu case

As the influenza pandemic threat level climbs toward an all-out 6, a growing list of countries is considering or issuing Mexico-related travel advisories and new airport screening procedures begin in Mexico, one of the biggest questions being asked about swine flu is:

Why is it hitting Mexico harder than anywhere else in the world? In other words, why are there more cases and, particularly, more deaths in Mexico when the flu to date seems relatively mild in the US, Canada and Europe?

Apples to apples

But even posing that question isn't so simple, explains Timothy Rodwell, an epidemiologist in the Division of Global Public Health at the University of California-San Diego.

"You have to compare apples to apples," he says.

Dr. Rodwell is currently working with a team of global public health and infectious disease experts from UCSD, alongside public health partners in Mexico, to monitor and treat influenza A(H1N1), as the new virus is now being called by the World Health Organization. The team is focusing on vulnerable patient populations in Tijuana and Ciudad Juárez, and they are seeking funds from the US National Institute on Drug Abuse to set up a rapid response surveillance program as soon as possible. 

Here, Dr. Rodwell outlines the three main points that would be necessary to make a comparison between the impact of swine flu in Mexico and in any other country.

1: Having enough cases

Many people will die "by chance alone," Dr. Rodwell said, and until you have accumulated a representative sample of confirmed swine flu cases, "you can't technically say what the true mortality rate in Mexico is yet."

So while the WHO has stated that the "more severe" cases in this outbreak have come from Mexico, and while there were 12 confirmed deaths in Mexico as of Thursday night, and 1 confirmed death in US, that could simply be a matter of how many cases have been collected so far in each country.

"Right now the US is too early in the outbreak to be fair about the comparison," he said. "Until you've collected enough cases, you can't make an accurate estimate of mortality in the US or Mexico."

2: The number of confirmed cases

The high number of deaths reported in Mexico are people who "could have died," but did not definitively die from swine flu. For example, someone who died of a respiratory illness in Mexico in the last two weeks might have had the influenza A(H1N1) virus - but nobody can be sure until that virus is isolated from their lungs.

By Tuesday, a 152-person death toll was being reported in news stories. That numbers referred to "probable" swine flu cases. On Tuesday night news broke that only 7 of those 152 deaths had been confirmed so far as being positive for swine flu.

The question to sort out, Dr. Rodwell said, is, "the number of people who normally die of respiratory illness" vs. the number of "mortalities [that] are associated directly with the virus."

3: Knowing what the patient population is like

Making an accurate comparison between the virus' impact in Mexico and in any other country also requires having an accurate picture of the diseased population in those countries.

A patient population is identified by its age structure, gender, and other diseases that might make an individual more likely to die--and have nothing to do with the virus. For example, if all the flu patients in one country were older or had HIV, then that country would have a higher number of flu deaths than a country where all the patients were young and healthy. Basically, if you are going to compare mortalities in two different countries, the population of sick individuals in those countries also has to be comparable.

"Right now", Dr. Rodwell said, "we don't really know if the population that is sick with flu in Mexico is similar to the population that is sick with flu in any other country. What that means is that it is difficult to make an ‘apples to apples' comparison between mortalities in Mexico and the US until we have more information."

Possible Mexico-specific factors

Dr. Rafael Lozano, a professor at the Institute for Health Metrics and Evaluation at the University of Washington, offered several observations about the situation in Mexico based on his former work as the General Director for Health Information in Mexico's Ministry of Health from 2001 - 2007.

"It's important that a true diagnosis" be reached, "that's based on laboratory tests," he said.

One problem he underscored in Mexico is that of people "arriving late" to the hospital. Hospitals had been notified by Mexico's national center for epidemiology (CENAVECE) to look out for patients with a combination of flu and pneumonia symptoms in a document dated April 18. A media blitz that raised public consciousness about the virus and its symptoms started gaining speed on Friday, April 24. 

The symptoms themselves might not have initially been cause for alarm, said Dr. Lozano. "If you have a cold, if you have a fever, you stay home and you try to auto-medicate," he said.

Felicia Knaul, an economist at the Fundación Mexicana para la Salud, also pointed out the tendency in Mexico to head first to the pharmacy, rather than to a doctor.

"What does the average Mexican do when they get sick? They go to the pharmacy and they self-medicate. More so when you think you have a cold.

"I think by the time you realize your are quite sick, it's quite far along," she added.

People with symptoms, "probably [went] very late" for treatment, said Dr. Lozano. "That's my guess."

Asked about Mexico City's notoriously polluted air quality and environmental risk factors that might aggravate flu symptoms and the severity of cases in Mexico City in particular, Dr. Lozano said:

"Of course, pollution is a risk factor. Of course, smog is a risk factor." But given that cases have also appeared in places such as San Luis Potosi and Veracruz, he added "I cannot make a co-relation with pollution."

Instead, Dr. Lozano referenced the four million people who use public transit daily in Mexico City. "That is like a country in movement," he said. "That's the main problem we have in Mexico City -- overcrowding -- a lot of contact."