The "war has changed" against Covid-19 because of the highly contagious Delta variant, the US Centers for Disease Control said, proposing a clearer message, mandatory vaccines for health workers and a return to universal masking.
An internal CDC document said the variant, first detected in India and now dominant across the globe, is as contagious as chickenpox and far more contagious than the common cold or flu.
It can be passed on even by vaccinated people, and may cause more serious disease than earlier coronavirus strains.
The document, entitled "Improving communications around vaccine breakthrough and vaccine effectiveness", said the variant required a new approach to help the public understand the danger - including making clear that unvaccinated people were more than 10 times more likely than those who are vaccinated to become seriously ill or die.
"Acknowledge the war has changed," it said. "Improve communications around individual risk among vaccinated."
A list of recommended prevention measures included making vaccines mandatory for health care professionals to protect the vulnerable and a return to universal wearing of face masks.
The CDC confirmed the authenticity of the document, which was first reported by the Washington Post.
While vaccinated people were less likely to become infected, once they contracted such "breakthrough infections" from Delta - unlike the case with earlier variants - they might now be just as likely as the unvaccinated to pass the disease on to others.
"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus," CDC head Rochelle Walensky said in a statement.
The CDC released data today from a study of an outbreak in Massachusetts in which it said three quarters of those infected had been fully vaccinated. That study had played a pivotal roll in a CDC decision this week to again recommend that vaccinated people wear masks in some situations, Ms Walensky said.
In parts of the world where large numbers of people have yet to be vaccinated, the Delta variant has led once again to surging death rates and hospitalisations.
WHO urges action to suppress Covid before deadlier variants emerge
The Delta variant is a warning to the world to suppress the virus quickly before it mutates again into something even worse, the WHO said.
"Delta is a warning: it's a warning that the virus is evolving but it is also a call to action that we need to move now before more dangerous variants emerge," the WHO's emergencies director Michael Ryan told a press conference.
WHO chief Tedros Adhanom Ghebreyesus added: "So far, four variants of concern have emerged -- and there will be more as long as the virus continues to spread."
Dr Tedros said that on average, infections increased by 80 percent over the past four weeks in five of the six WHO regions.
Though Delta has shaken many countries, Dr Ryan said proven measures to bring transmission under control still worked - notably physical distancing, wearing masks, hand hygiene and avoiding long periods indoors in poorly ventilated, busy places.
"They are stopping the Delta strain, especially when you add in vaccination," he said.
"The virus has got fitter, the virus has got faster. The game plan still works, but we need to implement and execute our game plan much more efficiently and much more effectively then we've ever done before."
Even in rich countries that were among the first to roll out big vaccination campaigns, cases have surged.
Nearly a third of US adults have yet to get a first shot. Areas where vaccination rates are low have seen sharp rises in cases in recent weeks, and authorities fear hospitalisations and deaths are not far behind.
Top US infectious diseases specialist Dr Anthony Fauci told Reuters he expected that vaccines, which so far have received only emergency approval, could begin getting full regulatory approval in August, and that this could help persuade more people to get vaccinated.
In Britain, where the Delta variant caused a sharp surge in infections in recent months despite one of the world's fastest inoculation campaigns, a panel advising the government said immunity from vaccines was likely to wane over time, meaning vaccination campaigns would probably last for years.
On Tuesday, the CDC, which had advised vaccinated Americans months ago that they no longer needed to wear masks, reversed course, saying even the fully vaccinated should wear face coverings in situations where the virus was likely to spread.
Yesterday, US President Joe Biden urged local governments to pay people to get vaccinated and set new rules requiring federal workers to provide proof of vaccination or face regular testing, mask mandates and travel restrictions.
"The main thing that does change (because of Delta) is that masks will still be used and that in countries where this requirement has been lifted, it will have to be re-introduced,"said Carlo Federico Perno, head of microbiology and immunology diagnostics at Rome's Bambino Gesù Hospital.
Delta not specifically targeting children: WHO
Meanwhile, the WHO said the Delta variant is not specifically targeting children.
The WHO's Covid-19 technical lead Maria Van Kerkhove said the evidence showed that the variant was rather being transmitted among people who were socially mixing.
"Let me be very clear: we are not seeing the Delta variant specifically target children," the US expert told a press conference.
The UN health agency said work was under way to gain a better understanding of the dynamics of Delta and why it is more transmissible.
"There was some suggestion that the variants were specifically targeting children, but that actually is not the case. What we are seeing is that the variants will target those who are socially mixing," Dr Van Kerkhove said.
"What we do see is that the variants that are circulating will infect people if they are not taking the proper precautions," she said, referring to measures such as physical distancing and avoiding gathering in poorly-ventilated, crowded indoor spaces.
The WHO has published a plan detailing ways in which schools can reopen and stay open in safety.
"But we do really need to drive down transmission in the communities to make sure that they can open safely," Dr Van Kerkhove said.