WASHINGTON (REUTERS) - The United States on Monday (June 14) crossed the grim milestone of 600,000 Covid-19 deaths, according to a Reuters tally, as slowing vaccination rates threaten the Biden administration target of having 70 per cent of US adults receive at least one shot and 160 million fully inoculated by July 4.

The early success of the US vaccine roll-out has had a huge impact on the pace of Covid-19 fatalities in the country.

It took 113 days to go from 500,000 total US Covid-19 deaths to 600,000 - the second slowest 100,000-death jump since the pandemic began. The nation went from 400,000 to 500,000 deaths in just 35 days.

"My heart goes out to those who've lost a loved one.... We have more work to do to beat this virus and now's not the time to let our guard down," said US President Joe Biden on the sidelines of Nato meetings in Brussels, urging people to get vaccinated.

The US seven-day Covid-19 death average has fallen by almost 90 per cent from its peak in January. The country reported 18,587 coronavirus-related deaths in May - about 81 per cent less than in January, Reuters data showed.

While the epicentre of the pandemic has shifted to places like Brazil and India in recent months, the United States remains the hardest-hit nation in terms of cumulative deaths.

But the country has so far vaccinated 166 million adults with at least one dose, according to the US Centres for Disease Control and Prevention (CDC), although the rate of shots administered has dropped significantly from a mid-April peak.

The average seven-day Covid-19 hospitalisation number has also plummeted since April due to vaccinations. On June 2, total hospitalised patients fell below 20,000 for the first time since June 24, 2020.

However, hospitalisation among teenagers has increased as more easily transmitted virus variants began to spread, according to recent CDC data.

The rate of hospitalisation due to Covid-19 increased among adolescents aged 12 to 17 in April to 1.3 per 100,000 people from a lower rate in mid-March, the agency reported.

Overall, daily new Covid-19 cases have also been dropping since March, with the country reporting the lowest number of cases per capita in May of this year, according to a Reuters analysis.

With vaccinations down to about 1.10 million doses per day last week - some 67 per cent lower than the highest seven-day rate - the Biden administration and state governors have come up with all manner of incentives to get unvaccinated people to roll up their sleeves. These include free childcare and rides to vaccination centres, extended Friday night hours at pharmacies and the chance to win US$1 million or college scholarships in a lottery.

As of Sunday, nearly 52 per cent of the US population has received their first vaccine dose, according to the CDC.