The delta variant of the coronavirus is likely the most common strain of COVID-19 in the United States, according to estimates released by the Centers for Disease Control and Prevention.

The CDC estimated that 51.7% of new cases for the two-week period ending July 3 were linked to the delta strain while 28.7% of new cases were linked to the alpha strain, which has been dominant in the U.S. throughout the course of the pandemic. The tallies are projections using case modeling and official breakdowns will be released in full later this month.

Some areas of the country, however, have seen COVID-19 cases linked to the delta variant skyrocket, particularly in states with lower vaccination rates. More than 80% of new cases in the region encompassing Kansas, Missouri, Nebraska and Iowa were predicted to be linked to the strain, the CDC said. 

Earlier this week, California officials said the delta variant was the most dominant strain identified in new cases, accounting for more than 35% of all infections by June 21. The strain was linked to just 5.6% of cases in May.

In response, public health officials have urged Americans to get vaccinated.

The World Health Organization also announced that the delta strain was the “most transmissible” one that health officials had identified so far and lambasted vaccine rollout efforts in low-income nations which had propelled a rapid rise in global cases.

The delta strain has already affected some countries’ COVID-19 response plans. 

In Australia, millions of residents in Sydney have seen lockdown orders extended amid a troubling chain of cases linked to a man who was infected with the delta variant. 

Bangladesh has extended its current lockdown into next week as hospitals on border areas struggle with an influx of patients infected with the delta variant.

Fiji reported a record 636 COVID-19 infections and 6 deaths on Monday, which caused the mortuary at the Pacific island’s main hospital to fill to capacity. Since the pandemic began, Fiji has reported a total of 39 deaths, but most have come since the emergence of the delta variant in April.

And while 95% of new cases in the U.K. are linked to the delta variant, the government announced plans to test the nation’s high vaccination rate with an almost full reopening of society on July 19. 

Studies have shown the Pfizer, Moderna and Johnson & Johnson vaccines were all effective against the strain in varying capacities. The Biden administration on Tuesday announced a new, summer-long push to vaccinate Americans who are still hesitant to get inoculated, including going door-to-door in some areas.