The Executive Director of the World Health Organization's Health Emergencies Programme has strongly criticised countries who have not shared their excess Covid-19 vaccine doses with developing nations.

Speaking at the MacGill Summer School, Dr Mike Ryan said millions of people around the world do not have any protection from the virus and that if those in developed countries are getting boosters, or are "at the top of the queue then the queue just gets longer".

Dr Ryan was critical of pharmaceutical companies who he said are making profit by manufacturing doses for countries who have "two or three" times the supplies they need and are not sharing with the WHO's COVAX programme.

In the last four-six months, developed countries made pledges in terms of vaccines they would donate to COVAX but only 15% of them were delivered, he added.

Dr Ryan said: "Pledges don't make donations" and "you can't stick pledges in people's arms".

The gap between the rhetoric and the reality on the planet is "very stark" and there are excess doses of vaccines that have not been delivered, he said.

Giving booster jabs when some people in the world are not vaccinated is not an "either/or" situation, he also said, and that it is possible to do both.

Dr Ryan - who has led the global response to Covid-19 - said it is about "Big Pharma" and geopolitics - "putting profit before people and before our planet".

Criticising politicians, he said many governments fear making the decision to send vaccines to poorer countries because of "retribution" from people in the future.

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Professor of Experimental Immunology at Trinity College Dublin Kingston Mills said it is not about "Ireland handing over a million doses" but about support.

India had the capability to manufacture "billions of doses of vaccines", he said, but the world needed to finance poorer countries in order to do this.

Prof Mills also told the summer school said that administering booster vaccines is going to "get us out of this hole we are in".

He pointed to the efficacy of studies in Israel that show immunity against Covid-19 went back up to 90% after a booster vaccination was administered.

Prof Mills said the propensity for variation in this virus "took us by surprise" and that emergence of variants - Delta in particular - made it more difficult for vaccines to give protection.

Waning immunity is a fact, he added, and booster vaccines would help.

Prof Mills said he expects numbers of positive Covid-19 cases in the UK to come down in the coming weeks, because the extraordinarily high figures now mean there is a natural immunity being built up, and that is good news for Ireland.

Professor Mary Horgan said that Ireland is in a time of uncertainty in the pandemic.

The President of The Royal College of Physicians told the MacGill Summer School that "just when you think we are getting there" in terms of the end of the pandemic, the virus "resets the goalposts".

She said every single tool available must be used to get through this winter.

Prof Horgan said that with such a high vaccination rate in Ireland, people get a false sense of security in relation to Covid-19, but the basics, such as hand hygiene and mask wearing are "so, so important".