CDC: Mexico's novel flu outbreak mirrors other hot spots

Jun 4, 2009 (CIDRAP News) – In an update on novel influenza developments in Mexico, the US Centers for Disease Control and Prevention (CDC) reported today that patterns look like those in the United States and other nations, with cases leveling off nationwide and areas of localized transmission.

The latest analysis of the disease in Mexico appears in tomorrow's issue of Morbidity and Mortality Weekly Report (MMWR). The Pan American Health Organization (PAHO) reported yesterday that Mexico has had 5,563 novel flu cases, 103 of them fatal. The nation has the world's highest number of deaths and the second highest number of cases, behind the United States.

Other features of the novel flu outbreak in Mexico resemble those in the United States and other countries that have reported a number of cases, Anne Schuchat, MD, interim deputy director CDC's science and public health program, said today during a media briefing. The virus is circulating during a time when activity is usually low and appears to be sparing the elderly, she added.

"This is a very helpful summary of the country that first had the largest problem noted," Schuchat said.

Today's update from the CDC also describes the health actions that Mexico has taken to control disease and the surveillance activities it has used to track the spread. Epidemiologic evidence suggests that Mexico's novel flu outbreak peaked in late April.

In the La Gloria, Veracruz, outbreak, which first prompted Mexico in mid April to explore outbreaks of acute respiratory illnesses, laboratory officials determined that the high number of illnesses in the small community were caused by a range of viruses, including seasonal influenza A (H3N2), seasonal influenza B, adenovirus, and novel H1N1, the CDC reported.

In the meantime, Mexico was investigating clusters of rapidly developing severe pneumonia in Mexico City, San Luis Potosi, and other locations. The two events prompted a national surveillance committee on Apr 17 to intensify surveillance for acute respiratory diseases and severe pneumonia. Mexico's health ministry also launched outbreak investigations with help from global health organizations.

Within 4 days, the surveillance system detected increased pneumonia hospitalizations in young adults in Mexico City, which led to the collection of isolates that were analyzed by laboratories in Canada and the United States and confirmed as novel H1N1 influenza.

A few days later, Mexico's epidemiology directorate established an Internet-based reporting system, which published its first report on Apr 26.

A surge in testing increased the daily load at Mexico's national laboratory from 30 to 900 specimens.

Of the 5,337 laboratory-confirmed cases covered in the report, nearly 98% were in people age 60 or younger. Nearly 56% of deaths occurred in patients between ages 30 and 59.

Mexico activated its pandemic plan on Apr 24, which led to school closures in Mexico City, a media campaign that taught the public how to avoid transmission, and a host of social distancing measures. On May 11, the government reopened the schools and education officials advised that parents and teachers identify sick students and close classrooms and schools when illness clusters surfaced.

At airports in Mexico, officials are still screening passengers for respiratory illnesses, and thermal scanners are in use at the Mexico City International Airport.

In an editorial note that accompanied the report, the CDC said that the large number of off-season novel flu outbreaks occurring around the world is a pattern that has been seen before with sustained transmission of other novel influenza A strains.

Circulation of the new virus in the southern hemisphere at the start of the region's annual influenza season in a population without much immunity to the virus suggests that the novel H1N1 strain might dominate during the coming months, the CDC reported.

CDC. Update: novel influenza A (H1N1) virus infection, Mexico, March-May, 2009. MMWR 2009 Jun 5;58(21):585-9 [Full text]

See also:

Jun 3 PAHO update

May 1 CIDRAP News story "Mexico's first swine flu case surfaced in mid-March"

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