by Crystal Phend, MedPage Today Senior Staff Writer

The pandemic H1N1 influenza virus continues to disproportionately attack the young, the CDC warned.

Children and adults under age 25 have accounted for 53% of hospitalizations for laboratory-confirmed H1N1 and 23.6% of related deaths since Sept. 1, the agency reported at a press briefing.

Seniors, on the other hand, have accounted for just 7% of H1N1-confirmed hospitalizations and 11.6% of deaths from the virus based on data from 27 and 28 states, respectively.

“This is really, really different than what we see with seasonal flu,” said Anne Schuchat, MD, director of the CDC’s National Center for Immunization and Respiratory Diseases, who led the briefing.

Seasonal flu typically causes 60% of its hospitalizations and 90% of fatalities in those 65 and older, she noted. “It’s almost completely reversed here,” a pattern that matches what was seen in the spring.

Schuchat reemphasized that physicians shouldn’t take “No” as the answer when using the influenza rapid diagnostic test.

“You can’t really rely on that. The accuracy of the rapid tests when they show up a negative is not that good,” she said.

Physicians should trust their clinical judgment in administering antivirals early to severely ill patients with flu-like symptoms or those with known risk factors for more severe disease, including pregnancy and asthma — without waiting for laboratory results or trusting a negative rapid result, Schuchat said.

Pregnancy, in particular, has been a risk factor for which the CDC said it has met resistance in its recommendation to treat with antivirals.

“Completely healthy pregnant women have been coming down with horrible, horrible illnesses [with the pandemic H1N1, including the need for] intensive care unit courses, and, tragically, more deaths that we’ve been hearing about recently,” Schuchat said.

Deaths in the young population were also more common in children with neurologic conditions, such a cerebral palsy and muscular dystrophy, although there have been some otherwise healthy children who have had the same grave outcome.

One reason behind the “overwhelmingly serious complications” seen in some healthy young people is that the virus can cause pneumonia if the infection spreads to the lower respiratory tract, which is difficult to treat even with antivirals, Schuchat said.

Another problem seen in some of the cases is that bacteria, particularly staph, can creep in after the influenza has weakened the immune system and cause serious pneumonia, she added.

Already this fall, 292 deaths have been reported by the 28 states reporting laboratory-confirmed H1N1 cases to the CDC. Schuchat said the number of cases likely reflected under-reporting, but that the ages of affected patients was probably a good reflection of practice.

The age breakdown was:

* 3.1% ages 4 and under
* 13.7% ages 5 to 18
* 6.8% ages 19 to 24
* 32.5% ages 25 to 49
* 32.2% ages 50 to 64
* 11.6% ages 65 and older

Over the same Sept. 1 to Oct. 10 period, there were 4,958 laboratory-confirmed H1N1-related hospitalizations reported to the CDC by 27 states.

The ages of those hospitalized were:

* 19% ages 4 and younger
* 25.3% ages 5 to 18
* 8.7% ages 19 to 24
* 24% ages 25 to 49
* 15% ages 50 to 64
* 7.2% ages 65 and older

These statistics support the CDC’s recommendations targeting younger populations for vaccination efforts, Schuchat said.

She noted that although the distribution of H1N1 vaccine still seems to be smaller than demand in most areas, the number of doses available for states to order rose from 9.8 million last week to 12.8 million, and the number actually ordered by the states roughly doubled.

“It probably feels like a slow start for a lot of people,” Schuchat said. “But we’re still really at the beginning [of the flu season]. We’re in it for the long haul.”

Based on the 1957/1958 pandemic flu, a second wave of infection might be expected in the spring, she noted. “At CDC, we’re planning a long response.”

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