Public-health officials and lab researchers are this week engaged in a frantic battle to contain the escalating outbreak of bird flu in Asia, and to minimize the risk of it developing into a human flu pandemic.
The first cases of apparent human-to-human transmission of the avian flu virus were reported in Vietnam on 1 February, heightening public-health concerns. As officials struggle to contain the avian outbreak by orchestrating the massacre of chicken flocks, researchers around the world are preparing the way for the mass production of a vaccine that could help to contain a human pandemic.
Individual human-to-human transmission of the virus doesn't necessarily mean that it has evolved into a strain that will transmit readily enough to cause a significant outbreak, never mind a pandemic. But should a pandemic occur, many experts say that current public-health plans are unlikely to prevent large-scale loss of life. Previous human flu pandemics, such as the 1918 outbreak that killed about 40 million people, originated in birds.
The present outbreak of the highly pathogenic H5N1 virus — named for the subtypes of its key surface proteins, haemagglutinin and neuraminidase — has ripped through flocks of domestic poultry in southeast Asia and spread to at least 14 people, killing 11 of them (see ‘Lack of infrastructure hampers virus monitoring’).
In efforts to curb the spread of the virus, the eight countries affected so far (see map) have slaughtered 20 million chickens. Experts say that culling infected birds is the only way to extinguish the virus before it infects more people. This can reduce the risk that avian and normal human flu meet and exchange genes in an individual, creating a strain with the ability to spread easily between people (see ‘Mixed messages’). Officials don't put much stock in vaccinating the region's vast chicken flocks, especially as this would not eliminate infectivity — healthy birds that are still shedding the virus would undermine the effort.
The World Health Organization (WHO) is working on the first stage of a five-point action plan for dealing with pandemic influenza, says Klaus Stöhr, project leader of the organization's global influenza programme. This involves seeking to contain the avian outbreak while monitoring human-to-human transmission.
The US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, a laboratory at St Jude Children's Research Hospital in Memphis, Tennessee, and the National Institute for Biological Standards and Control in London are working with the WHO to prepare a human vaccine. They expect a prototype H5N1 vaccine strain to be engineered shortly and safety-tested in ferrets within two months. This will then be handed to manufacturers for clinical testing, which should take up to another two months. If the virus starts spreading between people, mass production would begin, which would take an extra four to six months.
The lengthy production schedule means that in the worst-case scenario, the first wave of a pandemic could almost be over before a vaccine is ready, some experts warn. “Most of the damage will have been done by the time a vaccine is widely available,” says Rino Rappuoli, head of research at Chiron Vaccines in Siena, Italy, one of the major companies in Europe working on pandemic flu vaccines. Even so, a vaccine might be useful in fighting any subsequent ‘waves’ of the pandemic.
Stöhr concedes that “vaccines will not be ready for those at the epicentre of any pandemic”. Antiviral drugs will be the first line of medical intervention, he says, but these may also be in short supply. Preliminary tests done by the CDC have shown that the H5N1 virus is resistant to a traditional group of drugs, including amantadine and rimantadine, that block virus replication.
A newer and more expensive class of drug that cripples the neuraminidase enzyme in flu viruses is effective against H5N1 strains. The most common version of these drugs is oseltamivir (Tamiflu), made by Swiss-based drug company Roche. But the company won't say how many doses of Tamiflu it has or reveal its capacity to manufacture more, says spokesman Terence Hurley. He claims that Roche is cooperating with the WHO and offering Tamiflu at a reduced price for emergency use in poor countries.
Other experts doubt whether stocks of Tamiflu would be sufficient in the face of a pandemic. “It's in very limited supply and would quickly run out,” says epidemiologist Ira Longini at Emory University in Atlanta, Georgia, who has modelled Tamiflu's potential impact. Prompted by the severe flu season this winter, the CDC is already building up a significant stockpile of Tamiflu. Japan and some European countries are also negotiating with Roche to get stocks of the drug.
Meanwhile, efforts in southeast Asia are aimed at lowering the rate of transmission to humans by culling chickens as quickly and safely as possible. Stöhr is concerned that many of those doing the culling don't have the recommended protective clothing, and aren't being vaccinated against the circulating strain of normal human flu, or taking prophylactic antiviral drugs. These steps would reduce the chances of the avian flu combining with human flu strains in their bodies. “We need to get rid of the natural reservoir of H5N1, but we need to do it safely,” says Stöhr.
Some researchers hope that the current outbreak will add impetus to their calls for governments to be better prepared. They would like to see improved surveillance of circulating strains of animal flu, large-scale manufacture of vaccines to possible pandemic strains before human-to-human transmission is seen, or stockpiling of antiviral drugs.
Many countries are drafting such schemes. The German health authorities, for example, are finalizing a plan that would routinely prepare prototype vaccine strains based on subtypes of the haemagglutinin surface proteins circulating in animals or birds. “This could save months in vaccine preparation if a pandemic threat arises in the future,” says Reinhard Kurth, president of the Robert Koch Institute in Berlin.
The Canadian government has already implemented part of its pandemic influenza plan: it has a contract with Quebec-based Shire Biologics that requires the company to be ready to manufacture a pandemic vaccine. The company could produce 6 million doses per month once it is supplied with the prototype vaccine strain, says Health Canada spokesperson Emmanuel Chabot in Ottawa, Ontario.
Other countries should invest in similar schemes to ready themselves for future flu pandemics, experts say. “We are going to face this over and over again,” predicts public-health researcher Scott Layne of the University of California, Los Angeles.boxed-text
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Abbott, A., Pearson, H. Fear of human pandemic grows as bird flu sweeps through Asia. Nature 427, 472–473 (2004). https://doi.org/10.1038/427472a
Nature Medicine (2007)
Nature Medicine (2004)