Elderly More Likely to Die From H1N1 Influenza

By Fran Lowry
Medscape Medical News

November 12, 2009 — Mortality rates from H1N1 influenza show a J-shaped curve, with those aged 70 years and older having the greatest risk for death, according to the results of a retrospective analysis published online November?12 in The Lancet.

"In April, 2009, the first cases of influenza A H1N1 were registered in Mexico and associated with an unexpected number of deaths," write Santiago Echevaria-Zuna, MD, from the Instituto Mexicano del Seguro Social (Mexican Institute for Social Security), Mexico City, Mexico, and colleagues. "At present the pandemic has spread to more than 168 countries. We therefore need to stay alert — especially in countries with similar sociodemographic characteristics to Mexico, which might share conditions that could potentially contribute to H1N1 mortality."

The aim of this study was to report the timing and spread of H1N1 influenza in Mexico and investigate protective factors and risk factors for infection, severe disease, and death.

The investigators analyzed information from the influenza surveillance system from April 28 to July 31, 2009 for patients with influenza-like illness who attended clinics that were part of the Mexican Institute for Social Security network.

They found that the first large outbreak of H1N1 flu in Mexico affected the Mexico City metropolitan area, San Luis Potosi, and Zacatecas in April, and it lasted until June 6, with a peak number of cases reported on May 2.

During this first outbreak the mortality rate was high and almost all deaths occurred in the Mexico City area, where most teaching hospitals are located. This was before the population had been informed about H1N1 influenza symptoms and health services were not yet prepared for this new disease, the authors explain.

A second large outbreak occurred in southeast Mexico during June and July.

In all, 63,479 cases of influenza-like illness were reported, and 6945 (11%) cases of H1N1 were confirmed. Of these, 6407 (92%) were outpatients, 475 (7%) were admitted and survived, and 63 (<1%) died.

Young People Most Likely to Be Infected

Infection was transmitted mostly among young people, with those aged 10 to 39 years being the most affected. This age group accounted for 3922 cases (56%).

However, H1N1 affected people in the older age groups more severely. The investigators found that mortality rates showed a J-shaped curve. The greatest mortality risk was in those aged 70 years and older (10.3%). Mortality rates in other age groups were 5.7% for 60- to 69-year-olds); 4.5% for 50- to 59-year-olds); 2.7% for 40- to 49-year-olds; 2.0% for 30- to 39-year-olds; 0.9% for 20- to 29-year-olds; 0.2% for 10- to 19-year-olds; 0.3% for 1- to 9-year-olds; and 1.6% for infants younger than 1 year.

The analysis showed that fever, cough, headache, muscle aches, and rhinorrhoea were the main symptoms of H1N1 influenza. Dyspnea, tachypnea, cyanosis, and being confined to bed were prognostic factors for hospital admission and death. Patients with chronic illness had an increased risk for death; reported chronic diseases of those who died were hypertension, diabetes mellitus, and obesity.

Seasonal Influenza Vaccine Lowered Infection Risk

The authors also report that the risk for infection was lowered by 35% in those who had been vaccinated for seasonal influenza. They suggest that the seasonal vaccine, which includes H1N1 components, could have benefited the Mexican population, which has been getting the vaccine since 1977.

"The high incidence of infection in young people could show not only their different exposure related to their daily activities but also that people older than 60 years might have some immunity against the H1N1 virus," the authors write. However, they acknowledge that such possible protection is controversial.

Pregnant women accounted for 4 (6.3%) of 63 deaths. None of these women had received antiviral drugs during the first 48 hours of the onset of their symptoms and none had received influenza vaccine. "Treatment should begin immediately after onset of symptoms in this group, and vaccination during pregnancy is not contraindicated and therefore can be considered," the authors write.

Limitations of the study include provision of information sources by staff who have different training for handling H1N1 influenza infection, and incomplete data, which can occur even when the most advanced epidemiologic surveillance systems are used, the authors point out.

The authors conclude that although the H1N1 virus has spread to 168 countries, it has not reached the dimensions of the 1918 influenza pandemic, and some researchers believe that it will not, given the information on the virus available up to now. But they caution, "This pandemic might not be the one we expected; however, the virus is evolving and the threat continues."

Rapid Evolution of Knowledge

In an accompanying editorial, V. Alberto Laguna-Torres, MD, from the US Naval Medical Research Center Detachment in Lima, Peru, and Jorge Gomez Benavides, MD, from San Marcos University, in Lima, Peru, write that the rapid evolution of knowledge about the H1N1 pandemic has allowed medical groups to establish better capabilities with which to face it.

They add that the likely protection from seasonal influenza vaccine is clearer now than it was previously. "This vaccine fails to protect against the pandemic influenza virus in any age group. However, data show relative protection for people who were exposed to H1N1 strains during childhood before the 1957 pandemic."

The editorialists support the study's conclusion. "Decisions based on preliminary results and limited sources have to be made, and sometimes there is no time to wait for the pandemic to end to have stronger information," they write. "Currently, we know that the pandemic has not reached the dimensions of its predecessor in 1918, but the scientific knowledge has evolved faster than before, probably because of global and online communication."

The authors and editorialists have disclosed no relevant financial relationships.

Lancet. Published online November 12, 2009.