But they work best in people who are not yet sick but who have been exposed to the virus or may be exposed before the vaccine can develop effective protection.

A vaccine developed for one locality may not be protective in another locality, because of antigenic variation.

No public health measures have been undertaken or vaccines developed because of the rarity of human infection.

The scientists discover that "the vaccine developed previously for the avian strain will only provide partial protection," the plan states.

Some vaccines developed for cat coronavirus actually made the disease worse, he said.

Most healthy adults who get the vaccine develop protection to most or all of these types within 2 to 3 weeks of getting the shot.

But because H.I.V. eventually causes AIDS in virtually all untreated infected individuals, a vaccine recipient might develop AIDS years later.

Laboratory studies suggest that the vaccine developed previously for the avian strain will only provide partial protection against the new virus.

Singapore licenses a new vaccine, called Rotarix, developed by drug manufacturer GlaxoSmithKline (GSK) to protect infants against rotavirus gastroenteritis.

That means five of every 100 given the vaccine will not develop any immunity and will be susceptible to developing polio.