Washington

Trials of a new version of the smallpox vaccine have been halted because of a rare side effect, raising concerns about the vaccine's suitability for widespread use.

On 13 April, Acambis in Cambridge, UK, said that it had stopped recruiting patients into a large clinical trial of the vaccine because three people in the trial had developed myopericarditis — swelling of the heart muscle and surrounding tissue.

The US government has already ordered millions of doses of the new vaccine, called ACAM2000. But the latest finding makes it unlikely that it will be given to civilians unless there is an emergency.

The United States began a civilian vaccination programme in 2002. So far, about 40,000 emergency workers have been given an older version of the vaccine to prepare them for a bioterrorist attack. But the programme has moved far more slowly than was intended, largely because of people's concerns about side effects.

Myopericarditis was not seen during the large-scale smallpox eradication campaign of the 1960s and 1970s, which used the version of the vaccine called Dryvax. But the side effect has emerged in new clinical trials of Dryvax. The US Department of Defense reports that, since December 2002, 77 of 615,149 military workers who took Dryvax developed myopericarditis. In the Acambis trials, which compared Dryvax with ACAM2000, both versions of the vaccine seemed to cause the condition.

“It is still a big question whether this is something really new or whether these things happened before, but were not noted,” says Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland.

Fauci says that ACAM2000 could still be given to civilians in an emergency. But health officials are unsure how to continue tests of the new vaccine. It is also not clear how efforts to license the vaccine will proceed. Fauci says that it may become necessary to stockpile a milder version of the smallpox vaccine — modified vaccinia Ankara — which is also being tested.