Oct 16, 2009 (CIDRAP News) – The susceptibility of some young, healthy people to severe illness with pandemic H1N1 influenza marks a striking difference from the pattern of disease seen in seasonal flu epidemics, the World Health Organization (WHO) said today.
The factors that increase the risk of severe illness in previously healthy people remain unknown, the WHO said in reporting on the results of a 3-day conference on the features of severe H1N1 cases. The meeting involved about 100 clinicians, virologists, and other experts at the Pan American Health Organization headquarters in Washington, DC.
The ability of the virus to make young, healthy people dangerously sick has been noted for months, but the WHO put new emphasis on the phenomenon today. At the same time, the agency said pregnant women, children younger than 2 years, and people with chronic lung disease face the greatest risk of severe illness.
In a statement, the WHO said the experts confirmed that the vast majority of patients around the world experience an uncomplicated flu-like illness and recover within a week, even without treatment.
Patients hard to treat
But concern now focuses on "small subsets of patients who rapidly develop very severe progressive pneumonia," the agency said. "In these patients, severe pneumonia is often associated with failure of other organs, or marked worsening of underlying asthma or chronic obstructive airway disease."
These patients are hard to treat, which suggests that emergency rooms and intensive care units will bear the heaviest burden during the pandemic, the statement said. That conclusion matched the message from several medical journal reports published in the past week on hospitalized H1N1 cases.
Primary viral pneumonia is the most common finding in severe cases and often causes death, the WHO said. However, bacterial infections have been found in about 30% of fatal cases—more common than previously recognized.
Data from animal studies also show the virus's ability to cause severe pneumonia. "This virus really likes the lower respiratory tract," said the WHO's Dr.Nikki Shindo at a press teleconference today. "That means this virus is likely to cause viral pneumonia."
Physicians who have managed severe cases "agreed that the clinical picture in severe cases is strikingly different from the disease pattern seen during epidemics of seasonal influenza," the WHO statement said. "While people with certain underlying medical conditions, including pregnancy, are known to be at increased risk, many severe cases occur in previously healthy young people. In these patients, predisposing factors that increase the risk of severe illness are not presently understood, though research is under way."
In a separate pandemic update today, the WHO noted that about a third of intensive care unit patients with H1N1 in Australia and New Zealand had no predisposing conditions. Likewise, Canadian and Mexican researchers who recently reported on severe cases were "impressed" by the number that occurred in previously healthy people, the agency said.
The latest US figures suggest that an even higher proportion of patients hit hardest by the virus were previously healthy. The Centers for Disease Control and Prevention reported this week that 45% of about 1,400 adult H1N1 patients who were hospitalized had no preexisting health problems.
In severe cases, patients usually begin deteriorating about 3 to 5 days after their first symptoms, the WHO statement said. Many of them then slip into respiratory failure, requiring admission to an ICU and ventilatory support. Some patients don't respond well to conventional ventilatory support, making treatment even harder.
Known at-risk groups
Of groups with conditions that raise the risk of severe illness, conference participants agreed that three lead the list: pregnant women, especially in the third trimester; children under the age of 2 years, and people with chronic lung disease, including asthma, the WHO reported.
Disadvantaged populations, such as minority groups and indigenous people, also are disproportionately subject to severe disease, the WHO said. The reasons are not clear, but possibilities include lack of access to care and an increased prevalence of conditions like asthma and diabetes.
The statement also noted that obesity—especially morbid obesity—has been present in many of the severe H1N1 cases, but its role remains poorly understood.
More support for antivirals
On the brighter side, the meeting pointed up a growing body of evidence that prompt treatment with the antiviral drugs oseltamivir and zanamivir is helpful, the WHO said.
"We have increased evidence that timely antiviral treatment really helps to decrease the severe disease," said Shindo at the press conference.
Where the virus is circulating, clinicians should base antiviral treatment decisions on epidemiologic and clinical findings and not wait for lab test results, she said. "The message for clinicians is, don't miss this opportunity for early treatment."
Shindo said the WHO has shipped antivirals from its stockpile to 72 countries so far.
Oct 16 WHO report on clinical consultation
Oct 16 WHO weekly update on pandemic
Aug 28 WHO briefing note on lessons from recent outbreaks