Members of a Red Cross team don protective clothing before heading out to look for suspected victims of Ebola, in Mbandaka, Congo. Picture: Karsten Voigt/International Federation of Red Cross and Red Crescent Societies via AP

Geneva  - An immunization campaign to stem the Ebola outbreak in Congo started on Monday, World Health Organization (WHO) chief Tedros Adhanom Ghebreyesus said in Geneva.

"I'm pleased to say that vaccination is starting as we speak," Tedros said in a speech to the annual WHO meeting of more than 190 member countries.

The public-private GAVI vaccine alliance, which funds the effort, said that more than 7 500 WHO-supplied vaccine doses are available in Congo.

Ebola is a highly infectious disease that causes a fever that often leads to massive internal bleeding and is often fatal.

The Ebola shots will be administered using the so-called ring method, which targets people who have come in direct contact with patients, as well as the contacts of these primary contact persons.

"Health workers will be the first to receive the vaccine today, as they are the ones most likely to be exposed to the Ebola virus," said GAVI chief executive Seth Berkley.

At the WHO meeting, Germany pledged 5 million euros (5.9 million dollars) to fund the fight against the outbreak.

"We must do our utmost to prevent the disease from spreading," a spokeswoman for German Health Minister Jens Spahn said.

So far, 22 people in the central African country have been confirmed to have Ebola, according to Congo Health Ministry data from Monday. There are another 27 suspected and possible cases, for a total of 49.

Among the confirmed and suspected cases, the death toll stands at 27.

Getting the Ebola vaccines to where they are needed is challenging because the outbreak has hit some of the most remote parts of the Congo.

"They will need to be transported to one of the most remote parts of the [country], where there are next to no paved roads, electricity or telecommunications, at a temperature of minus 60 to minus 80 degrees Centigrade," GAVI chief Berkley said.

After the outbreak spread from rural areas in the north-western Equateur province to its capital Mbandaka, the WHO said Friday that the risk of a country-wide spread had become very high and that the risk of a spread to neighbouring countries was high.

The UN health agency did not declare an international health emergency. During the last major Ebola outbreak in 2014, 11 000 died when the virus spread across the West African countries of Guinea, Liberia and Sierra Leone. The WHO's then-leader Margaret Chan was criticized for the agency's slow response to the eastern African epidemic.

"It's concerning that we now have cases of Ebola in an urban centre [in Congo], but we are much better placed to deal with this outbreak than we were in 2014," Tedros said.

GAVI said that 300 000 immunization doses are available, including those in or en route to Congo. Although the vaccine is unlicensed, it was shown to be safe and effective in Guinean trials.