Health inspectors examine lab that made vaccine batch

· 'Hardy' virus may have survived production process meant to kill it
· Infection from company's animals could have spread to farm, says professor
Health and safety inspectors were yesterday examining the laboratory complex identified as a possible source of the foot and mouth outbreak.

The virus strain involved has been identified as 01 BFS67, which was isolated during the 1967 outbreak of the disease and which had been used to manufacture a vaccine batch at the Merial Animal Health laboratory in Pirbright in July. A spokesman said the vaccine was being developed for use in another country.

John Oxford, professor of virology at Barts and the Royal London hospitals said one theory that should be investigated was that the live virus produced in the manufacture of the vaccine had not been completely killed during the process. If so, it could have retained its ability to infect animals.

There had been incidents in other countries where an incompletely killed virus had been the source of infections, he said. "In the past there have been problems, not in this institute, but in different places around the world. It has proven quite difficult to kill the virus 100% ... It is a very hardy virus," he said. "[In this case] it could have produced a bit of infection in their [Merial's] own animals, which then spread to the farm."

To produce the vaccine, scientists begin with a sample of the strain that is causing the infection. Vaccines are specific to individual strains.

First, laboratory workers allow the vaccine to replicate in cultures of mammal cells - usually hamster cells. The next step is to kill the virus without changing its molecular appearance too much. The idea is that it will no longer be able to infect cells and replicate, but will still be recognised by a cow's immune system, so that an immune response mounted to the vaccine will be effective against the real virus. This is done with two chemicals that cause the outer coat of the virus to stick to its genetic material inside. That disables the virus by preventing it from entering cells or replicating.

Next, researchers check, while still under maximum biosecurity conditions in the laboratory, that the virus really is dead. This is crucial, because if it retains its ability to replicate, the vaccine will cause the disease. Once the researchers are satisfied that a batch of vaccine is safe it can be taken out of the lab and tested on cows or sheep to ensure it elicits the correct immune response.

"It is that sort of thing that could get you into problems if the thing is not dead," said Prof Oxford. He speculated that Merial might have carried out such tests on animals in a paddock at Pirbright and a virus not fully dead might have spread from there. If this is correct it will be good news, because the virus will be less virulent than normal. It should spread more slowly and will be easier to contain.

In the meantime, identifying the strain and likely source of the virus so quickly should greatly help efforts to control the disease. It means the government can prepare stocks of vaccine specific to the current outbreak in case it spreads. It should take about three days to produce vaccine in the quantities necessary.