Baxter says cell-based H5N1 vaccine did well in trial

Jun 12, 2008 (CIDRAP News) – A pharmaceutical company is reporting good results in the first clinical trial of an H5N1 avian influenza vaccine that uses a whole, killed H5N1 virus grown in cell culture—a combination of techniques that entails some risk but may boost immune response and shorten production time.

Low doses of the vaccine, made by Baxter Bioscience, generated immune responses against the vaccine strain of H5N1 in most volunteers and also induced responses against two other H5N1 strains in many of them, according to a report published today in the New England Journal of Medicine (NEJM). Formulations without an adjuvant (immune-boosting chemical) yielded the best responses, and the vaccine caused few side effects.

Most H5N1 vaccines tested so far have been produced by the conventional method of growing a genetically modified virus in chicken eggs, which takes several months. The Baxter vaccine was grown in Vero cell cultures, a method said to offer increased speed and flexibility.

In addition, Baxter took the unusual step of using a whole, "wild-type" (but inactivated) H5N1 virus in the vaccine, instead of a genetically modified, weakened version. Some evidence suggests that whole-virus vaccines against novel flu strains may be more immunogenic than split-virus or subunit vaccines, and producing them takes fewer steps, according to the authors.

However, growing large amounts of a wild-type H5N1 virus poses a risk that the virus might somehow escape and trigger devastating outbreaks in poultry. Consequently, Baxter had to build a high-security (biosafety level 3) facility to grow the virus.

The vaccine, called Celvapan, is made in a cell-culture system in Bohumil, Czech Republic, Baxter officials said in a Jun 11 news release. "Baxter's Vero cell manufacturing process is more rapid due to its ability to use a 'native' virus that does not need to be modified to allow growth in chicken eggs, therefore accelerating vaccine production," the company said.

The vaccine uses a clade 1 strain of H5N1 virus that was isolated in Vietnam in 2004, known as A/Vietnam/1203/2004.

Six formulations tested
The researchers, led by Hartmut J. Ehrlich, MD, and Noel Barrett, both of Baxter, recruited 284 adults (aged 18 to 45) for the double-blind trial, conducted at one site in Austria and two in Singapore. The volunteers were randomly assigned to receive one of six vaccine formulations: 3.75, 7.5, 15, or 30 micrograms (mcg) with an alum adjuvant, or 7.5 or 15 mcg with no adjuvant. Each volunteer received two doses, 21 days apart.

The team assessed immune responses primarily by measuring neutralizing antibodies, defining seroconversion as at least a fourfold increase in antibody titer 21 days after the first and second doses. The scientists assessed antibody reactions not only to the vaccine strain, but also to a 2005 clade 2 strain from Indonesia and a 1997 clade 3 strain from Hong Kong.

The highest rates of seroconversion were seen in volunteers who received the vaccine without adjuvant: 69.0% with the 7.5-mcg dose and 68.3% with the 15-mcg dose (after two doses). Similarly, those who received nonadjuvanted vaccine had the highest rates of immune response as defined by a virus neutralization titer of 1:20 or more. With the 7.5-mcg dose the rates were 40.5% after one dose and 76.2% after two doses; with the 15-mcg dose the corresponding rates were 39.5% and 70.7%.

The formulations without adjuvant also generated high rates of cross-reactivity against the clade 3 virus, as 76.2% of the 7.5-mcg group and 78.0% of the 15-mcg group had a neutralizing antibody titer of 1:20 or more after two doses. Cross-reactivity against the clade 2 strain was lower: 45.2% at 7.5 mcg and 36.6% at 15 mcg. Statistical analysis showed that the no-adjuvant doses induced significantly higher immune responses to both the vaccine strain and the other two strains than the adjuvanted versions did.

The vaccine had a safety profile like that of H5N1 subunit vaccines described previously, the authors report. The most common reaction at the injection site was pain (9% to 27% of volunteers, depending on the formulation), and the most common systemic reaction was headache (6% to 31% of vaccinees). Fever occurred in only 2% to 7%.

The results, the authors conclude, show "that a broadly reactive immune response to clade 2 and clade 3 of H5N1 virus can be obtained with the use of a low-dose clade 1 vaccine without adjuvant." Because the 7.5-mcg formulation without adjuvant yielded the best immune response, Baxter has chosen it for further development, they add.

Cell culture's debut hailed
In an accompanying perspective article in NEJM, Peter F. Wright, MD, cites the introduction of a cell-culture–derived flu vaccine as perhaps the most important innovation represented by the Baxter report. Conventional egg-based technology requires decisions in February concerning the next season's vaccine strains, he notes. But with cell-culture vaccines, "this schedule could be altered to permit incorporation of late-emerging threats."

A graphic accompanying Wright's commentary indicates that cell-culture technology would permit mass production of a vaccine within 12 weeks from the time a pandemic flu virus is identified, versus 22 weeks for egg-based technology.

Wright, a professor of pediatrics at Dartmouth Medical School in Hanover, N.H., also cites the possibility that using whole virus to make the vaccine resulted in higher immunogenicity. This would be consistent with trial results for the swine influenza vaccine created in 1976. But it is not safe to assume that the Baxter vaccine is more immunogenic than the US-licensed H5N1 vaccine—a subvirion vaccine—because different immunogenicity assays were used for the two products, he adds. Further, the whole-virus vaccines in 1976 triggered the strongest side effects in children, so the new vaccine may have a different safety profile in children than in adults.

Wright also says the report raises the question whether large-scale production of vaccine from a wild-type H5N1 virus would be safe. The experience in using wild-type poliovirus to make vaccines is reassuring, he writes, adding that the closed systems used to protect vaccines' sterility "greatly limit the opportunities for spread."

Pros and cons of whole-virus vaccines
John Treanor, MD, who conducts viral vaccine research at the University of Rochester in New York, called the Baxter report "intensely interesting" on multiple counts, including the use of a whole, wild-type virus and cell culture production. Treanor is a professor of medicine and of microbiology and immunology.

Commenting by e-mail, he said the question of whole-virus versus subunit vaccines has raised an "intriguing discussion." Some data suggest that whole-virus vaccines yield better immune responses than subunit vaccines do in hosts with no previous exposure to the virus (as in a pandemic), which makes some intuitive sense, he said. However, this advantage doesn't hold true with seasonal flu viruses, and there is good evidence that whole-virus vaccines cause more side effects in young children.

Whole-virus vaccines might be easier to make, because they require fewer steps, Treanor said. "On the other hand, most of the current manufacturers don't make whole-virion vaccines, so they would have to significantly change their current process to go back to a whole-virion approach," he added. Given the pros and cons, it's hard to say which approach is better, he said.

"Then the really interesting thing that Baxter has done is to use the wild-type cleavage sequence," Treanor added. He explained that H5 viruses are highly pathogenic in birds because their hemagglutinin (HA) surface protein can be cleaved easily by proteases founds in all bird tissues, permitting the virus to bind to and enter cells. "Viruses that have a highly cleaveable HA must be handled under high containment because of the risk that the virus might escape and cause outbreaks of lethal influenza in chickens—an agricultural nightmare," he said.

The standard approach is to use reverse genetics to replace the amino acid sequence at the HA cleavage site with a sequence from a less pathogenic H5 virus, Treanor said. He added that he's not sure why Baxter chose to use the wild-type sequence, but this approach eliminates the need to use reverse genetics, "and since that requires licensing, maybe that is what they're trying to avoid."

Still a long way to go
Another infectious-disease expert, Michael T. Osterholm, PhD, MPH, welcomed the findings but cautioned that the world is still a long way from having the practical capability to quickly make an effective vaccine in large enough quantities to protect most people in the event of a pandemic.

"I think it's an important step forward in that we're now reporting results from cell culture, but the pandemic clock is ticking, and there remains considerable confusion about what the different serologic results mean in terms of human protection," said Osterholm, director of the University of Minnesota Center for Infectious Disease Research and Policy, which publishes CIDRAP News.

He called Wright's commentary a thoughtful analysis of the state of flu vaccine research, but said, "His comments . . . fail to take into account the supply and manufacturing facility requirements to make vaccines for the next pandemic. Even if we had a vaccine today that was an ideal candidate for use in the next pandemic, the international capacity to make vaccines, package them, and provide all of the syringes, needles, and other vaccine support materials for a worldwide campaign are extremely limited."

For example, he said it now takes up to 3 years or even longer for the delivery and installation of equipment to bottle and fill vaccines

Baxter's news release said the European Medicines Agency accepted the company's vaccine for licensing review earlier this year. The statement also said a federally supported trial of the product is under way in the United States.

Ehrlich HJ, Muller M, Oh HML, et al. A clinical trial of a whole-virus H5N1 vaccine derived from cell culture. N Eng J Med 2008 Jun 12;358(24):2573-84 [Full text]

See also:

Jun 11 Baxter news release

November 2007 CIDRAP News special report: "The pandemic vaccine puzzle, part 6: Looking to novel vaccine technologies"

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