快猫短视频

A POLIO outbreak in Haiti and the Dominican Republic two years ago was caused by mutant forms of the virus used in polio vaccine that might have acquired the ability to spread from person to person by swapping genes with viruses in the gut.

The weakened form of the polio virus used in the oral vaccine has about 50 changes in its RNA compared with the original virus, making it less dangerous and less likely to spread. It鈥檚 known that a few random reversals of these changes can make it deadly again.

But the latest study raises the frightening possibility that the vaccine strain can also regain the ability to circulate among people more easily than anyone thought. 鈥淚t demonstrates clearly that the vaccine virus can spread from person to person,鈥� says Olen Kew from the Centers for Disease Control and Prevention in Atlanta, Georgia.

His team鈥檚 analysis of the viruses that caused the outbreaks suggests that they all stem from a vaccine given to a child in Haiti late in 1998. Mutated forms began circulating among people who hadn鈥檛 been vaccinated, reaching the Dominican Republic by 2000. Some became virulent enough to cause polio鈥�21 people fell ill during 2000 and 2001, and two died.

It turns out that the mutant forms had swapped RNA sequences with related enteroviruses that can infect our guts. But the researchers can鈥檛 be sure if this gene swapping was a cause of the outbreaks.

The outbreaks were made worse because Haiti had relaxed its polio vaccination programme five years earlier. 鈥淚t鈥檚 a warning that you need to have good coverage to prevent vaccines from running away like this,鈥� Kew says. It also shows how difficult it will be to eliminate the polio virus entirely.