People should still practise physical distancing and other coronavirus mitigation measures even as the US vaccination programme makes headway, global scientists said in a report published on Wednesday in the Nature peer-reviewed journal Scientific Reports.
Researchers at Reykjavik University, University of Lyon, University of Southern Denmark and University of Naples Federico II evaluated real-time flight data for US domestic travel and – using a mathematical formula – suggest that vaccination alone will not be enough to promptly end the current wave or fend off a future one in the US.
The report comes a day after the US announced 50 per cent of its adult population is fully vaccinated.
But that is only 129 million people out of a general population of 330 million, and the rate of vaccinations is slowly decreasing, according to a Bloomberg vaccine tracker.
Until there is a higher number of fully vaccinated people and a greater level of herd immunity, precautionary steps should still be taken by everyone, the scientists said.
"Our analysis clearly demonstrates that continued vigilance is needed regarding social distancing and other non-pharmaceutical methods in the US, since not everyone can be vaccinated at once and because there is a considerable time lag from vaccination to immunity," said Dr Anna Sigridur Islind, who was part of the study.
The scientists are essentially advocating a continued all-round approach to the pandemic, employing physical distancing and other Covid prevention measures while vaccines continue to be administered.
Herd immunity, however, is a much-debated topic because it is a moving target.
"I always say there's this elusive, somewhat mystical terminology of 'herd immunity' and 'herd immunity threshold'," top US infectious diseases expert Dr Anthony Fauci told a Boston radio station in early May.
Dr Fauci has estimated herd immunity to be 70 per cent to 85 per cent for the US.
Others say new vaccine-resistant Covid-19 variants may force the proportion of fully vaccinated people to be higher to reach herd immunity.
The US Centres for Disease Control and Prevention said on May 13 that fully vaccinated people do not need to physically distance or wear a mask indoors, or outdoors in many cases, which led to the removal of many social distancing requirements across the country.
According to a tracker from The New York Times, more than half of US states have fully reopened.
The remaining states have varying reopening timelines and mask rules for indoor spaces, showing an uneven approach to the pandemic in the US.
The findings are "especially important in a pandemic as complex as Covid and that has had such an uneven public health response, with huge variability in vaccination rates, social distancing regulations and other measures," said Dr Maria Oskarsdottir, another researcher involved in the study.
There is still a federal requirement to wear a face mask when travelling on planes, buses or trains in the US. Airlines, however, have removed social distancing measures such as blocking off middle seats on planes.
US Secretary of Homeland Security Alejandro Mayorkas on Tuesday addressed the frustration of travellers hoping to return to normal during the mask mandate, which will be in place through September.
"These are rules that are designed for their very protection and so that we can ensure that the success that we enjoy today in travelling will actually be replicated each and every day," Mr Mayorkas said.
More than 37 million Americans are expected to travel this Memorial Day holiday weekend.