The World Health Organization's (WHO) top scientist said Monday that COVID-19 vaccines will probably be available in December or early 2021 for submission to regulators for approval.

Asked by Anadolu Agency about a possible timeline for vaccines to combat the pandemic, Dr. Soumya Swaminathan said: "It's very encouraging to see the progress in the clinical trials that are happening all around the world."

He added: "As you know, we have about 40 vaccine candidates now in some stage of clinical trials, and 10 of them are in the Phase 3 trials, which are the late-stage clinical trials, which will tell us about both the efficacy and the safety."

He said that the earliest date for a vaccine to come onto the market to submit to regulators "is starting from December of 2020, into the early part of 2021."

Dr. Mariangela Simao, the WHO's head of access to medicines and health products, said that the vaccine developed would have to be licensed in the country where the trial took place.

"And we also have the manufacturing capacity it will need to be available to be used at the country level. So many, many steps need to be taken after its Phase 3 trial actually ends," said Simao, adding that the WHO is working very closely with national regulatory authorities.

Earlier, WHO Director-General Dr. Tedros Ghebreyesus spoke out against the "concept of reaching so-called herd immunity by letting the virus spread."

He said herd immunity is a concept used for vaccination in which a population can be protected from a particular virus if an immunization threshold is reached.

Tedros said that never in the history of public health had herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic.

He cited how immunity against measles would require about 95% of the population to be vaccinated.

"The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated," said Tedros.

"It's scientifically and ethically problematic. First, we don't know enough about immunity to COVID-19.

"Most people who are infected with the virus that causes COVID-19 develop an immune response within the first few weeks, but we don't know how strong or lasting that immune response is, or how it differs for different people. We have some clues, but we don't have the complete picture."

Tedros said there is no known immunity to the novel coronavirus.