CDC heartened by initial safety reports on H1N1 vaccine

Dec 4, 2009 (CIDRAP News) – Preliminary studies indicate that the pandemic H1N1 vaccine is just as safe as seasonal flu vaccines and is very unlikely to increase the risk of the paralytic condition that derailed the 1976 swine flu vaccination campaign, the Centers for Disease Control and Prevention (CDC) said today.

Also, the CDC reported today that H1N1 activity continues to decline, with 25 states citing widespread activity as of last week, down from 32 states the week before. The agency also said the cumulative total of available vaccine has reached 73 million doses, up 10 million from last week.

Data from two separate H1N1 vaccine safety monitoring systems have shown no increased risk for serious adverse events, compared with seasonal flu vaccination, the CDC said in a special report. And no cases of Guillian-Barre syndrome (GBS), the paralytic condition, have been linked to 438,000 vaccinations given to members of a network of eight managed care organizations.

At a news briefing, CDC Director Dr. Thomas Frieden called the safety findings very reassuring, though preliminary. "The likelihood that we'll have a 1976-like problem with this year's H1N1 vaccine is vanishingly remote," he said.

The 1976 vaccine was associated with an 8- to 10-fold increase in risk for GBS (to about 1 in 100,000 doses), Frieden said. "We're virtually certain that there isn't that level of increased risk now," he added.

He acknowledged that more time is needed to determine if there is a small increase in risk, because it can take 4 to 6 weeks after vaccination for the syndrome to appear, and the number of people who were vaccinated that long ago is still relatively small.

Safety findings detailed
The preliminary vaccine safety report, published as an early release by Morbidity and Mortality Weekly Report (MMWR), covers roughly the first 2 months of pandemic H1N1 immunization. It includes 3,783 reports received through the US Vaccine Adverse Event Reporting System (VAERS) and information on 438,376 people from eight managed care organizations included in the Vaccine Safety Datalink (VSD) system.

Though the rate of adverse events through VAERS was higher for the pandemic vaccine when compared with the seasonal vaccine, the CDC's vaccine experts did not detect any differences between the two vaccines in the proportion or types of adverse events reported.

An analysis of reports from VAERS, designed to record health events that occurred after vaccination and not the cause, found 204 serious adverse events for the pandemic vaccine and 283 for the seasonal vaccine. The overall event reporting rates were 82 per 1 million doses for the pandemic vaccine and 47 per 1 million for the seasonal counterpart. However, the CDC said the percentage of serious adverse events among all adverse events for the seasonal vaccine was slightly higher, 6.1%, than the 5.4% seen for the pandemic vaccine.

Of 13 post-vaccine deaths reported to VAERS for the pandemic vaccine, 9 patients had significant underlying illnesses and 1 death resulted from a motor vehicle crash. The CDC is reviewing autopsy results and death certificates for the other 3.

As of Nov 24, VAERS had received 10 GBS reports, plus an additional 2 that were flagged by medical officers who reviewed the data. Review of the patients' charts found that only 4 of the 12 met the formal GBS diagnostic criteria, and 4 others are under review. Of 19 anaphylaxis cases identified in the reports, 13 met the diagnostic criteria, and 5 had an anaphylaxis diagnosis on the medical record.

The CDC didn't identify any patterns in the other serious adverse events reported to VAERS after pandemic H1N1 immunization.

Meanwhile, the VSD system did not detect any GBS cases in those who had received the pandemic vaccine. Only one anaphylaxis case was observed, and the system did not flag increased rates for any other monitored conditions, which included several neurologic disorders, allergic reactions, and myocarditis.

The report says the VAERS and VSD findings are useful because clinical trials had limited ability to detect safety problems, given that sample sizes were small and the patient populations were healthy.

The CDC said the reporting rate after pandemic vaccine administration might be higher than for the seasonal vaccine because reporting is enhanced and the public is more aware of publicity surrounding the new vaccine.

Though the VSD system did not indicate any increases in serious health events above background rates, the CDC cautioned that vaccination rates in the managed care populations may not have reached a level high enough to detect small increases in risk for rare diseases.

The agency also said that, while the number of GBS cases seems smaller than what would be expected for a population of 30 to 40 million people, comparisons are difficult because of possible underreporting to VAERS and possible differences between vaccinated and unvaccinated populations.

H1N1 activity continues to drop
Meanwhile, the CDC's weekly Flu View update today says pandemic flu activity was still widespread in 25 states last week, compared with 32 states the week before. Seventeen states had regional activity, and the rest had only local outbreaks or sporadic cases.

The CDC's chart of lab-confirmed hospitalizations and deaths shows that the numbers for last week were less than half of the previous week's figures. However, the proportion of outpatient medical visits attributed to flu-like illness was 3.7% overall, still above the national baseline of 2.3%.

The percentage of deaths due to pneumonia and flu last week was 7.2%, just above the epidemic threshold of 7.1%, according to figures from the CDC's 122-Cities Mortality Reporting System.

Last week brought 17 more reports of pediatric deaths due to influenza, including 12 that involved confirmed H1N1 infections, the CDC said. That brings the total of H1N1 pediatric deaths to 210. The latest numbers signaled a decrease, as there were 35 pediatric deaths, including 27 from confirmed H1N1, the previous week.

Seasonal flu viruses remained very scarce last week. More than 99% of flu viruses that were subtyped were the pandemic strain. The CDC found only one H3N2 virus and 6 type B isolates.

Vaccination outlook
At the news briefing, Frieden was upbeat about the vaccination program, saying another 10 million doses should become available in the next week and there is evidence of continued strong demand for the vaccine.

"I think we're still in a period when we're changing gears, from a time when there wasn't enough vaccine to a time when it's increasingly available," he said.

"Doctors tell us there are still lots of people who want to get vaccinated and haven't gotten vaccinated yet," he added.

He cited Arkansas, Maine, and Rhode Island as states that have been particularly successful in vaccinating schoolchildren, who are included in the priority groups. In those states, "At least half of kids, and more in some areas, are being vaccinated. That's a tremendous accomplishment," he said.

He also said the number of obstetricians who are vaccinating their pregnant patients or at least know where their patients can get the vaccine is growing.

Frieden also said the supply of liquid oseltamivir (Tamiflu) for children is improving and more children with severe flu are receiving antiviral drugs. In most years the proportion of children who are receiving an antiviral by the time they are hospitalized for flu treatment is only 15% to 20%, but this year it's over 80%, he reported.

See also:

Dec 4 MMWR report on H1N1 vaccine safety

CDC's Flu View surveillance update

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