Smallpox immunity warning

An actress demonstrates the effects of smallpox

US researchers are warning people vaccinated against smallpox as children that they are unlikely to still be protected.

Smallpox was eradicated in the mid-1970s, but researchers warn bioterrorism fears mean mass vaccination should now be reconsidered.

But experts are divided over whether research, featured in New Scientist magazine, mean there should be a renewed mass smallpox vaccination programme.

In tests, doctors from Maryland found only 6% of over 600 microbiologists who were being re-vaccinated in the late 1990s were still immune to smallpox from their earlier vaccinations.

One in a million recipients is likely to die

Dr David Brown, PHLS

Smallpox is highly transmissible from person to person and around 33% of those who catch it die.

In America, around 60% of the population has had a smallpox vaccination, but most will be just as susceptible to smallpox as the 120m born since the government stopped the vaccination programme in 1972, the researchers say.

Britain also stopped its vaccination programme in the early 1970s.

Lack of protection

Michael Sauri, director of the Occupational Medicine Clinic in Maryland said: "The study is, to the best of my knowledge, the only one since eradication which tries to look at the durability of immunity.

"It's showing us that after 20 years immunity is not going to be there."

Bill Bicknell of Boston University, a former commissioner of the Massachusetts Department of Public Health thinks the Maryland research backs up his view that there should be mass smallpox vaccination.

He believes it is necessary in case terrorists use smallpox in attacks.

"It adds to the argument that you can't count on any protection we thought we had," he said.

"I'm not saying you just go straight in and vaccinate the population - you'd do it steadily in stages."

He recommends healthcare workers should be first, followed by volunteers screened to check they're healthy.

But many argue against mass vaccination.

Uncertain risk

Like many other countries, Britain currently prefers "ring vaccination", where only people in the area of an outbreak and people they are in contact with are immunised.

Dr David Brown, director of the UK's Public Health Laboratory Service Virus Reference Division, told BBC News Online: "It's generally agreed that you've got almost complete protection against smallpox if you were vaccinated in the last three years, but it decreases from that time.

He said the arguments around vaccinating because of bioterrorism fears centred around the risk of a smallpox attack contrasted with the risks associated with mass immunisation.

"What we're not certain of is what the risk of a bioterrorist attack is. There is a risk, but it's never been well defined."

"And if we did go for mass vaccination, we would have to go for multiple vaccination - every three to five years - for full immunity."

He added: "One in a million recipients is likely to die, and that's without considering cases infected with HIV."

Illness rates, he said, would be even higher.

The Maryland research was also published in Maryland Medicine.