THURSDAY, April 19, 2007 (HealthDay News) -- This week's approval by the U.S. Food and Drug Administration of a less-than-perfect vaccine against bird flu is only the beginning of the search for a truly effective shot against the virus, experts said.
Luckily, the annual search for a vaccine against seasonal flu is helping scientists as they seek immunization against its much deadlier cousin, H5N1 avian influenza.
"The two [efforts] go hand in hand," explained Dr. Roland A. Levandowski, chief of the influenza, SARS and other viral respiratory disease section at the National Institute of Allergy and Infectious Diseases. "You can't just all of a sudden manufacture an [avian] vaccine if you don't know how to make the seasonal vaccine. The hurdle for all vaccines is to demonstrate that they're safe and effective."
But big challenges remain.
According to Levandowski, there are vaccine products at all different levels of development for both seasonal and avian flu strains.
On Tuesday, the FDA approved the first bird flu vaccine for use in humans.
That vaccine, developed by drug maker Sanofi Aventis, is far from perfect, however, and was described as an "interim measure" by Norman Baylor, director of the FDA's Office of Vaccines Research and Review. According to the FDA, only 45 percent of people who received the vaccine produced infection-fighting antibodies to the virus.
That's much lower than the regular seasonal flu vaccine, making it unclear as to how effective the newly approved bird flu shot would be in the event of an epidemic.
"There's no way to know how effective this will be in practice," said Philip Alcabes, an epidemiologist and associate professor at the School of Health Sciences of Hunter College in New York City. "That's unlike the situation in seasonal flu, where we know from last year and the year before and the year before roughly how much protection you get from the vaccines. We have no way of knowing that from a new vaccine, because the epidemic hasn't happened."
It's also difficult to predict safety with any accuracy in a clinical trials setting. A 1970s vaccine developed for swine flu posed no problems in 100,000 volunteers but caused severe reactions in one in every 500,000 people once it went public, said Dr. Paul K. Carlton Jr., USAF (Ret.) and director of the Texas A&M Health Science Center Office of Homeland Security.
In the past two years, the H5N1 strain of avian flu has infected poultry throughout Southeast Asia, Central Asia, Africa and Europe, prompting the destruction of millions of birds. So far, more than 100 people have died worldwide from H5N1 infection, which has been spread through close contact with birds.
The big worry among health officials is that the virus will acquire the ability to jump easily between humans, leading to a pandemic and millions of deaths. Unlike the seasonal flu, humans have no immunity to bird flu.
Producing and distributing enough vaccine to protect large populations is a key problem. According to experts, avian flu vaccines are essentially made in the same way as the regular flu vaccine. Growing the viruses in eggs is the main method now, but companies and researchers are diversifying into other areas which might prove more reliable.
"There's been a push to try to reduce the reliance on eggs and use tissue cultures, which have been used a lot for other vaccines," Levandowski said. "There are also manufacturers who are trying to improve immunogenicity by developing adjuvant systems that can be used with a vaccine."
The H5N1 bird flu virus grows well in tissue culture, which is a real plus. But it has less hemoglutinin, the key immune system antigen, than other strains. "This is something that varies from virus to virus, but H5N1 was surprisingly lower compared to the seasonal vaccine," Levandowski said.
Those are some of the technical issues, but developing an avian flu vaccine also poses other challenges, not the least of which is not knowing which exact virus would cause a pandemic, should one occur.
"The biggest challenge is that we're not sure what's going to cause the problem," Carlton said. "When we say pandemic flu, it could be H5N1, it could be any of the H and N combinations."
Other experts feel that a pandemic is not an inevitability.
When government researchers last year tried to combine H5N1 with a common strain of flu that infects humans, they were unable to produce a strain that could be transmitted easily. That research offered some hope that a bird flu pandemic may not strike in the foreseeable future, if at all.
"I think it's very, very, very unlikely that there will be a pandemic or any kind of human epidemic with H5N1," Alcabes said. "But it seems that if being prepared isn't so hard and is not harmful, then why not be prepared. There are a lot of unknowns, and the biggest unknown is whether there even can be an epidemic of avian flu."
Visit the U.S. Centers for Disease Control and Prevention for more on bird flu.