A Mexican study has confirmed that younger people were more likely to become infected with the pandemic H1N1 influenza virus but that older people were more likely to die from it.
The majority of confirmed infections (56%) occurred in individuals 10 to 39 years old, with only 10.2% occurring in people 40 and older, according to Victor Borja-Aburto, PhD, of the Mexican Institute for Social Security in Mexico City, and colleagues.
"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," they wrote online in The Lancet.
However, the death rate was highest among those 70 and older, at 10.3%. The death rate was 2% or less for all age groups younger than 40.
The findings are consistent with previous epidemiological studies of the ongoing pandemic.
"Some researchers believe, with the information available up to now, that the present H1N1 inﬂuenza virus will not cause a pandemic on the scale of those during the 20th century," the researchers said. "This pandemic might not be the one we expected; however, the virus is evolving and the threat continues."
Borja-Aburto and his colleagues examined data obtained at Mexican clinics between April 28 and the end of July.
Over that time, 63,479 cases of influenza-like illness -- characterized by fever, cough, headache, muscular pain, and rhinorrhea -- were reported, with 6,945 cases of H1N1 confirmed.
Risk of confirmed H1N1 infection was significantly reduced in patients who had received a seasonal influenza vaccine in the past year (OR 0.65, 95% CI 0.55 to 0.77).
The researchers acknowledged that the potential for the seasonal vaccine to protect against the pandemic strain remains controversial and said that the association deserves further study.
Anne Schuchat, MD, director of the CDC's National Center for Immunization and Respiratory Diseases, has said seasonal flu vaccines are unlikely to provide meaningful protection against the pandemic strain. (See Older Population Might Have Immunity to H1N1 Swine Flu)
Of the confirmed cases, 92% received outpatient treatment, 7% were hospitalized and survived, and less than 1% (63 patients) died.
Mortality rates were low until age 40, after which they steadily climbed with increasing age.
Each day of delay in hospital admission was associated with about a 20% increased risk of death (OR 1.19, 95% CI 1.11 to 1.28).
Consistent with other studies, the presence of chronic diseases such as hypertension, diabetes, and obesity was linked to a greater risk of dying (OR 6.1, 95% CI 2.37 to 15.99).
Pregnant women accounted for 6.3% of the deaths from confirmed H1N1 infection, which is slightly lower than the 8% rate observed in the U.S. during a similar time frame.
"In Mexico, all pregnant workers were sent home during the peak of the pandemic, which probably accounts for this difference," the researchers said.
None of the pregnant women who died received antivirals during first 48 hours after symptom onset, as recommended by the CDC, and none had received a seasonal flu vaccine.
"Treatment should begin immediately after onset of symptoms in this group," the researchers said, "and vaccination during pregnancy is not contraindicated and therefore can be considered."
Borja-Aburto and his colleagues acknowledged that the study was limited by incomplete data and use of information provided by staff who were not necessarily trained to handle epidemics.
In an accompanying comment, V. Alberto Laguna-Torres, MD, of the U.S. Naval Medical Research Center Detachment in Lima, Peru, and Jorge Gomez Benavides, PhD, of San Marcos University in Lima and the Peruvian Ministry of Health, said the study was also limited because some patients eligible for treatment at clinics included in the study could have sought care elsewhere.
Influenza surveillance in general also has shortcomings because not all patients with flu-like illness seek treatment or are sampled during a pandemic, and diagnosis depends on access to laboratories, they added.
"All of these factors contribute to a bias in incidence rates," they declared.
There was no funding source for the study.
Both the study authors and the editorialists reported that they had no conflicts of interest.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner
The LancetSource Reference: Echevarría-Zuno S, et al "Infection and death from influenza A H1N1 virus in Mexico: A retrospective analysis" Lancet 2009; DOI: 10.1016/S0140-6736(09)61638-X.
The LancetSource Reference: Laguna-Torres V, Gomez Benavides J "Infection and death from influenza A H1N1 virus in Mexico" Lancet 2009; DOI: 10.1016/S0140-6736(09)61916-4.