The Delta variant-led fourth COVID-19 wave in United States seems to be ebbing, but possible emergence of mutations coupled with the onset of colder weather remains a concern. The seven-day average of daily cases in the latest surge as of Monday was at its highest even as most states were reporting a slow growth rate of the virus. Bhakti Hansoti, an associate professor in emergency medicine at John Hopkins University told news agency AFP she saw the US following a similar trajectory to India, where a third COVID-19 wave petered out after wreaking havoc for several months. She, however, emphasised the need to learn lessons from the fourth wave to prevent future surges.

Other experts say the fourth wave may still be far from over.

"If you look at the fall-winter wave, there were periods in which there was a steep exponential increase, and then it looked like it was falling -- and then there would be another increase," Angela Rasmussen, a virologist at University of Saskatchewan in Canada, told AFP.

Vaccination could also prove critical. With 54% of its total population fully vaccinated, the US is trailing behind countries like Portugal and the United Arab Emirates (UAE), which have fully vaccinated 81% and 79% of their populations, respectively.

The Joe Biden administration has announced a slew of measures to boost the vaccination campaign. It is also waiting for a nod from Federal Drug Administration(FDA) and Centers for Disease Control and Prevention (CDC) to provide booster shots of COVID-19 vaccines for added immunity.

CDC Director Dr Rochelle Walensky cited new data that showed that people who had not been vaccinated were 4.5 times more likely than the fully vaccinated to get infected, over 10 times more likely to be hospitalized and 11 times more likely to die.

In an opinion piece for medical journal Lanet, some of the world's leading scientists said booster shots were not needed yet despite the highly contagious delta variant.