* Outbreak unusual in that most victims are young adults
* Mass vaccinations scheduled in Congo, DRC and Angola
* Setback to global campaign to wipe out crippling disease
(adds global figures, background, byline)
By Stephanie Nebehay
GENEVA, Nov 11 (Reuters) - Polio has killed nearly 100 people, mainly young adults, in the Republic of Congo and paralysed more than twice as many in the past six weeks, the World Health Organisation (WHO) said on Thursday.
The crippling viral disease normally strikes children under five years of age, making the acute, fast-spreading outbreak unusual, the U.N. agency said.
"Most of the cases have involved young adults aged between 15 and 29. This illustrates that populations are at risk because they have not been exposed to a full immunisation," it said.
It marks the latest setback to a global campaign begun more than 20 years ago to wipe out polio, for which there is no cure, only preventive vaccines.
The death toll in the central African state is 97 with a further 226 people paralysed, and most victims are in the port city of Pointe Noire. The mortality rate is higher than normal for the disease, which attacks the nervous system.
The virus is now confirmed as having reached Congo from Angola after it also spread to the Democratic Republic of Congo earlier this year, WHO spokesman Oliver Rosenbauer said.
It comes from a strain from India, one of four remaining endemic countries -- where the virus survives and its spread has never been interrupted -- along with Afghanistan, Nigeria and Pakistan.
A first wave of mass polio vaccination campaigns, targeting 3 million people of all ages, is set to begin on Friday in Congo and parts of Angola and the Democratic Republic of Congo.
"This way we can be assured that everybody is reached, including young adults, whose immunity may be low," said Luis Sambo, WHO's regional director for Africa.
The WHO warned last month that a persistent outbreak of polio that began in Angola in 2007 could spread internationally due to poorly managed vaccination campaigns.
An epidemic this year in Tajikistan, a Central Asian state, accounts for 458 of 767 cases confirmed so far in 19 countries, Rosenbauer said. There were a 1,604 cases worldwide in 2009.
Pakistan is the only endemic country to have higher numbers of cases this year, with 111 against 76 at this time last year, he said. Record floods and poor sanitation are blamed.
Namibia had a polio outbreak among adults in 2006 but managed to stop its spread within 50 days.
The WHO, UNICEF, Rotary International and the U.S. Centers for Disease Control and Prevention have been working together since 1988 to eradicate polio, which infected at least 350,000 people in 125 endemic countries each year at the time. But their initial target of the year 2000 proved over-optimistic. (Editing by Jonathan Lynn and David Stamp)