Dr Mike Ryan likes to use a metaphor to make his point. Earlier this year, he compared booster jabs to handing out life jackets to those who already have one. The WHO Executive Director has now turned to geography to illustrate his perspective.

Asked by Prime Time about fatigue with public health measures, Dr Ryan said: "People feel that we climbed the mountain two or three times. We got the vaccines, and look where we are again."

But he warned that Europe is "approaching the worst days of pandemic again" and urged people to reduce social contacts and take vaccines when they are offered.

He warned that socialising in Ireland was at pre-pandemic levels.

"We are mixing together in a winter environment while the virus is still here."

Praising the Irish vaccine rollout, Dr Ryan implored those who remained unvaccinated to "do the maths".

"Look at your odds of being very, very sick or ending up with a ventilator. If you're not vaccinated, they are so much higher."

Dr Ryan said vulnerable and older people will benefit from an additional booster jab and, extending the climbing metaphor, added that the vaccine rollout has improved in the developing world.

"This isn't about one or the other - it's about getting the right level of priority. We are beginning to climb that hill now," he said.

"The WHO is still horrified by the gross inequity in vaccination access but that doesn't mean in a situation like Ireland if someone is offered an additional dose of vaccine because they're vulnerable that they should not take that dose."

Dr Ryan warned that countries needed to be cautious when considering introducing mandatory vaccination but conceded some "may feel they have no other option".

"I would advise countries considering mandatory vaccination to be sure that they have tried everything else. We don't want the lazy option here."

He said mandatory vaccination, "should be for the shortest possible period of time" and warned there was a fine line to balance public health with individual human rights.

"I think you have to be in a position where you have tried everything else and you're not making progress... where the potential health impacts in terms of deaths is so high that you have no other choice."

He added: "The best way to achieve high vaccine coverage is to communicate, to convince, to persuade. To show people the evidence and bring them along the way."