US also provided a boost to efforts to share out vaccines across the world by announcing it intended to join WHO's Covax initiative.
The United States has said it would resume its funding for the UN's health agency as President Joe Biden shifts towards greater international cooperation in the fight against Covid-19, while also launching a $1.9 trillion plan to tackle the pandemic at home.
"Under trying circumstances, this organisation has rallied the scientific and research and development community to accelerate vaccines, therapies and diagnostics," top US scientist Anthony Fauci told a WHO meeting via video-link, confirming that the US would continue to pay its dues to the organisation.
Biden was a fierce critic of Trump's approach to tackling the virus in the US, which with more than 400,000 people dead is the world's worst-hit nation.
The new president is seeking to vaccinate 100 million people in the next 100 days, increase the use of masks and testing, expand the public health workforce and offer more emergency relief to those struggling with the restrictions.
"For almost a year now, Americans could not look to the federal government for any strategy," said Jeff Zients, coordinator of the new Covid-19 task force. "As president Biden steps into office today, that all changes."
Virus cases are approaching 100 million globally, with more than two million deaths and many millions – from Beijing to Berlin – still living under lockdowns, curfews or other restrictions.
Europe has been particularly hard hit, though the Russian capital Moscow announced on Thursday it was lifting many of its harshest restrictions as Mayor Sergei Sobyanin expressed "cautious optimism" over the current figures.
We are currently facing four converging crises — COVID-19, the resulting economic crisis, climate change, and racial inequity. Today, President Biden took actions to combat these challenges. pic.twitter.com/091jmvCukk— The White House (@WhiteHouse) January 21, 2021
Wearing masks isn't a partisan issue — it's a patriotic act that can save countless lives. That's why I signed an executive order today issuing a mask mandate on federal property. It's time to mask up, America.— President Biden (@POTUS) January 21, 2021
More contagious coronavirus variants are travelling quickly around the globe, tempering optimism that mass vaccination campaigns would bring a swift end to the worst phase of the pandemic.
And the WHO has repeatedly warned that richer countries are hogging the vaccine, a point underscored by data from Africa suggesting the second wave is proving far more deadly than the first.
John Nkengasong of Africa Centres for Disease Control and Prevention said more infections meant more pressures on ill-equipped health systems, adding, "That also means you're overwhelming the ability of nurses, doctors to manage patients."
However, the US provided a boost to efforts to share out vaccines across the world by announcing it intended to join the Covax initiative, a pool of doses supplied by countries and companies.
Germany meanwhile confirmed it was willing to help Russia to develop its Sputnik vaccine, which has been rolled out by Moscow despite still being in clinical trials.
And early results from studies of the Pfizer-BioNTech vaccine suggested it would be effective against the British variant, which is fuelling a surge that has overwhelmed UK hospitals.
"When you go into a hospital ... in some cases it looks like a war zone," said the British government's chief scientist, Patrick Vallance.
Sigh of relief
The international community appeared to let out a collective sigh of relief as Washington vowed to re-engage with the World Health Organization.
"WHO is complete again," Ilona Kickbusch, the founding director and chair of the Global Health Centre in Geneva, told AFP.
"Everybody has wanted the US back in, and having such a multilaterally minded US administration ... is a gift to the world."
WHO chief Tedros Adhanom Ghebreyesus tweeted that in a conversation with Vice President Kamala Harris, "I thanked her & President @JoeBiden for their commitment to @WHO & global health," adding he was also grateful for their commitment to advancing women's health and climate action.
"This is a good day for WHO, and a good day for global health," Tedros said after Fauci's statement.
"We are all glad that the United States is staying in the family."
Beautiful moment between Dr Tony Fauci and @DrTedros:— James Chau (@jameschau) January 21, 2021
“ @WHO is a family of nations. And we are all glad that the United States 🇺🇸 is staying in the family. We are family 🌎 “@DrSenait @schwartlanderb @doctorsoumya @PeterASinger @takeshi_kasai @gabbystern @VBKerry pic.twitter.com/Z5e50p5wbb
Numerous countries also took the floor to celebrate the US' return.
Summing up the mood, British ambassador Julian Braithwaite hailed the US decision to "rejoin the family of nations committed to working together through the WHO," and to support "defeating this pandemic, the greatest challenge of our time."
Moussa Faki Mahamat, who chairs the African Union Commission, chimed in, tweeting that he was "delighted that Biden made the return of the USA to WHO one of his first decisions."