After declining fast for two months, the rate of Covid infections in the United States has leveled off since mid-June due to localised spikes in under-vaccinated regions of the country.

It comes as the highly contagious Delta variant continues to gain traction. It now accounts for 35.6% of sequenced cases in the past two weeks, according to data from the covSpectrum tracker.

Authorities have said it is poised to soon become the country's dominant strain.

The seven day average of new daily cases has been 3.5 per 100,000 people since June 16, according to the Centers for Disease Control and Prevention.

A clear divide has emerged across the country, with cases rising rapidly in communities that have low rates of vaccination.

The city of Springfield in the midwestern state of Missouri is experiencing a surge of 36.8 new cases per 100,000.

It has just 35% of its population vaccinated with one or more doses, a dashboard maintained by Covid Act Now showed.

On the other hand, Burlington in the northeastern state of Vermont is seeing just 0.9 new cases per 100,000, and has 71% of its population vaccinated with one or more doses.

Nationally, 54% of the population has received one or more doses while 46.1% of people have had two doses, according to the CDC.

The national immunisation drive peaked in mid-April, but has been declining since then.

Vaccine hesitancy is much higher in states that lean Republican than those that predominantly vote for Democrats.

Cases dropped steeply across the country from mid-April to the beginning of June, a period that coincided with spring weather and increased socialisation outdoors.

The summer heat however has brought more people indoors, where the primarily airborne respiratory illness mostly spreads.

Air conditioning contributes by drying out the air and making conditions more favorable for the virus to remain viable.