Twenty months into a pandemic that has claimed almost 770,000 American lives and nearly a year after Covid-19 vaccinations became available, US health experts warn a fifth wave of infections risks overwhelming health systems in the worst-affected states.

Fears of another surge in infections come on the cusp of the Thanksgiving holiday, when tens of millions of Americans will travel to reconnect with their families, some of them for the first time since the start of the pandemic.

But what was expected to be a celebration has become fraught with danger in some Midwestern states, where vaccination rates are low and Covid-19 cases are rising rapidly after a summer lull.

Even some northeastern states where vaccination rates are above 70 per cent, such as Maine and Vermont, are experiencing an uptick in case numbers, a trend that probably reflects breakthrough infections caused by waning immunity to Covid jabs, according to epidemiologists.

For now, at least, hospitalization rates in these states are not rising as rapidly as in the Midwest, suggesting vaccines remain a vital tool to prevent serious illness.