Study finds H1N1 pandemic in Mexico more deadly

Oct 6, 2011 (CIDRAP News) – Researchers who took a detailed look at 2009 H1N1 deaths in Mexico, where the virus was first detected, found that the new flu virus was more deadly than most seasonal flu epidemics and more lethal than in other countries, which confirms impressions early in the pandemic that the disease was worse in Mexico.

Investigators from the United States and Mexico compared excess mortality rates as well as years of life lost (YLL), a measure designed to show a clearer picture of the influenza pandemic through the integration of age effects and public health impact. The study appeared yesterday in Clinical Infectious Diseases.

Morbidity and mortality burdens of the 2009 H1N1 pandemic are still topics of debate, and pinning down the true health impact is difficult owing to differences in diagnostic testing and disease reporting. In 2010, a group that included some of the same authors as the current researchers came up with a new estimate of 2009 H1N1 deaths based on "years of life lost" (YLL), a measure that better reflects the shift of pandemic deaths toward younger age-groups.

That study, which appeared in March 2010 in Public Library of Science (PLoS) Currents, concluded that the 2009 H1N1 virus had a substantial health burden in the United States over the first few months of the pandemic, which justified efforts to protect the public with a pandemic vaccine.

In the new study, the researchers estimated excess mortality and YLL in Mexico using monthly age- and cause-specific death rates from January 2000 through April 2010 as well as population-based influenza surveillance.

Dr. Lone Simonsen, a study coauthor and epidemiologist at George Washington University, told CIDRAP News that the research group was able to conduct a detailed analysis of Mexico's pandemic mortality burden because the country has excellent and timely mortality records. Data for the pandemic time period was available earlier than that from other countries, and the group was able to access other health data, including age-specific death rates from lab-confirmed 2009 H1N1 cases from a private sector health system that covers about 40% of the Mexican population. To plot flu activity patterns, they used weekly data from a network of 38 labs located throughout Mexico.

The group found that the 2009 H1N1 virus caused a greater mortality burden in Mexico compared with other regions and countries, such as the United States, Europe, Australia, and New Zealand, where the impact is known. The pandemic was associated with 11 deaths per 100,000 population during three pandemic waves. For comparison, one study put the US pandemic death rate at 4 per 100,000 population.

The researchers estimated that the YLL was 445,000 during the same time period.

In addition, the investigators found that the pandemic mortality burden was 0.6 to 2.6 times that for seasonal flu but lower than during the severe 2003-04 Fujian A/H3N2 flu season.

Adults ages 20 to 59 were hit hardest by the pandemic, followed by young people 5 to 19 years old. They found that using lab-confirmed deaths to capture the mortality burden underestimated the impact of the disease on older people in 2009. They found that the system only captured 1 of 7 pandemic related excess all-cause deaths.

Their analysis found another wave of excess deaths that struck older people during the early months of 2010 at a time when pandemic flu was occurring but at a far lower rate. The group said it couldn't rule out the possibility that other respiratory pathogens such as respiratory syncytial virus or environmental factors could explain this wave of senior deaths.

Simonsen said the findings confirmed earlier suspicions that the disease was more deadly in Mexico, which is an area for further study. She also said the findings underscore an important public health message that pandemic severity can vary in different countries.

Researchers aren't sure why the virus was more deadly in Mexico, and the finding provides interesting research angles, she said. Some scientists have speculated that the virus that hit Mexico early could have been different from what circulated after that, though virologists haven't found much drift, Simonsen said. Other possibilities might include other cofactors, such as presence of pneumococcal bacteria that could have led to worse outcomes in pandemic flu patients. Or, differences in intensive care practices could have led to different survival rates of severely ill patients, she said.

The findings about Mexico's mortality burden seem to justify the intense measures the country's public health officials took initially, such as school closures and limiting public gatherings, to slow the spread of the virus, Simonsen said.

One lesson from the study for future pandemic planning is that officials should not count just deaths but instead consider the burden in terms of YLL to capture the unique flavor of pandemic influenza impact, Simonsen said, adding that another lesson is to not ease back on response measures if the first wave seems mild. "It is important to remember that even the devastating 1918 pandemic had a milder first wave that affected young adults," she said.

Charu V, Chowell G, Mejia LSP, et al. Mortality burden of the A/H1N1 pandemic in Mexico: a comparison of deaths and years of life lost to seasonal influenza. Clin Infect Dis 2011 Oct 5;985-93 [Abstract]

See also:

Mar 23, 2010, CIDRAP News story "Study: In life-years lost, H1N1 pandemic had sizable impact"

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