The World Health Organization does not expect widespread vaccinations against COVID-19 until the middle of next year, a spokeswoman said on Friday, stressing the importance of rigourous checks on their effectiveness and safety.

None of the candidate vaccines in advanced clinical trials so far has demonstrated a "clear signal" of efficacy at the level of at least 50% sought by the WHO, spokeswoman Margaret Harris said.

Russia granted regulatory approval to a COVID-19 vaccine in August after less than two months of human testing, prompting some Western experts to question its safety and efficacy.

U.S. public health officials and Pfizer Inc said on Thursday a vaccine could be ready for distribution as soon as late October. That would be just ahead of the U.S. election on Nov. 3 in which the pandemic is likely to be a major factor among voters deciding whether President Donald Trump wins a second term.

"We are really not expecting to see widespread vaccination until the middle of next year," Harris told a U.N. briefing in Geneva.

"This phase 3 must take longer because you need to see how truly protective the vaccine is and you also need to see how safe it is," she added. This referred to the phase in vaccine research where large clinical trials among people are conducted. Harris did not refer to any specific vaccine candidate.