Current data does not indicate that Covid-19 booster shots are needed, the World Health Organization has said.
The WHO added that the most vulnerable people worldwide should be fully vaccinated before high-income countries deploy a top-up.
"We're planning to hand out extra life jackets to people who already have life jackets, while we're leaving other people to drown without a single life jacket," WHO's emergency director Mike Ryan told reporters from the UN agency's Geneva headquarters.
"The fundamental, ethical reality is we're handing out second life jackets while leaving millions and millions of people without anything to protect them."
The comments came just before the US government said it planned to make the booster shots widely available to all Americans starting on 20 September as infections from the Delta variant of the coronavirus rise.
Yesterday, Minister for Health Stephen Donnelly said his department will be working with the HSE and the vaccine task force on plans for a booster shot roll-out.
The US had already authorised an extra dose for people with weakened immune systems. Israel has also begun administering third doses to citizens aged 50 and over.
WHO chief scientist Soumya Swaminathan, asked about the need for boosters to increase protection against the disease, told a Geneva news conference: "We believe clearly that the data today does not indicate that boosters are needed."
Further research was needed, she added.
WHO senior adviser Bruce Aylward, referring to booster shots being administered in high-income countries, told reporters: "There is enough vaccine around the world, but it is not going to the right places in the right order."
Two doses should be given to the most vulnerable worldwide before boosters are administered to those fully vaccinated, he said, adding: "We are a long, long way from that."
WHO chief Tedros Adhanom Ghebreyesus voiced outrage at reports that the single-dose J&J vaccine currently being filled and finished in South Africa was being shipped for use in Europe "where virtually all adults have been offered vaccines at this point".
"We urge J&J to urgently prioritise distribution of their vaccines to Africa before considering supplies to rich countries that already have sufficient access," he said.
"Vaccine injustice is a shame on all humanity and if we don't tackle it together, we will prolong the acute stage of this pandemic for years when it could be over in a matter of months."
Israel tightens restrictions as virus cases rise
New restrictions aimed at curbing the spread of Covid took effect in Israel, after the largely vaccinated country saw the highest daily infection rate since January.
The measures, announced Sunday, require vaccination certificates or negative coronavirus tests to enter a range of public spaces, including restaurants and bars, cultural and sports venues, hotels and gyms, the health ministry said.
The same applies to worshippers wishing to enter synagogues, mosques or churches with more than 50 people in attendance.
In addition, the capacity of stores, shopping malls and industrial parks will be limited to one person per seven square metres).
After its launch last December, Israel's widely praised vaccination drive helped to drastically bring down infections.
But that trend has since reversed, driven by the spread of the more contagious Delta variant of the virus, with restrictions that were lifted in June reimposed since July.
In recent weeks, the state has begun administering booster shots to Israelis aged 50 and over, while urging anyone aged 12 and older to get vaccinated.
About one million Israelis have not been vaccinated even though they are eligible.
According to the health ministry, more than 8,700 people tested positive for coronavirus yesterday, the highest number for a single day since January.
Prime Minister Naftali Bennett has been imploring Israelis to get vaccinated, warning of a possible lockdown that could affect the Jewish high holidays next month unless inoculation numbers rise.
Anti-mask mandate Texas governor tests positive for Covid-19
The Republican governor of Texas, who has been a forceful opponent of mask mandates, has tested positive for Covid-19, his office has said.
Greg Abbott, who is fully vaccinated, "has been testing daily, and today was the first positive test result," said a statement from his spokesman Mark Miner.
"The Governor will isolate in the Governor's Mansion and continue to test daily," the statement said, adding that Abbott is receiving an antibody cocktail treatment.
Mr Abbott is "in good health, and currently experiencing no symptoms," the statement said.
"Everyone that the Governor has been in close contact with today has been notified. Texas First Lady Cecilia Abbott tested negative."
Mr Abbott has been one of the most outspoken US governors against anti-Covid mask mandates. Texas has banned local governments in the southern US state and any entities that receive public funds from imposing vaccination requirements.
He has received pushback, with several Texas school districts imposing their own mask mandates against state law.
"Governor Abbott has put his own Republican primary politics before the public health since day one," former Democratic presidential candidate and former San Antonio mayor Julian Castro tweeted, along with a video from Mr Abbott's campaign of an indoor Republican Party event Monday night with dozens of unmasked attendees.
"I hope he recovers quickly. I also hope he will act more responsibly on behalf of Texas children and families."
"This virus is spreading like wildfire across Texas, including among children. I hope the governor now reconsiders his ban on mask requirements in schools," Mr Castro said in a separate tweet.
Pope Francis urges everyone to get vaccinated
Pope Francis has issued an appeal urging people to get inoculated against Covid-19, saying the vaccines could bring an end to the pandemic, but needed to be taken by everyone.
"Thanks to God's grace and to the work of many, we now have vaccines to protect us from Covid-19," the pope said in a video message made on behalf of the nonprofit US group the Ad Council and the public health coalition Covid Collaborative.
"They grant us the hope of ending the pandemic, but only if they are available to all and if we work together," he said.
Vaccines are widely available in mainly wealthier nations, but mistrust and hesitancy over the newly developed shots have meant that many people are refusing to take them, leaving them especially vulnerable as the Delta variant spreads.
By contrast, poorer nations still do not have access to large-scale vaccine supplies.
Medical experts have warned that ever-more dangerous variants might develop if the virus is allowed to circulate in large pools of non-vaccinated people.
Pope Francis was himself vaccinated in March, saying at the time that it was an ethical obligation.
"Vaccination is a simple but profound way of promoting the common good and caring for each other, especially the most vulnerable. I pray to God that everyone may contribute their own small grain of sand, their own small gesture of love," the pope said in his latest video message.
