After months of watching COVID-19 numbers in the United States fall, it’s no longer accurate to say the virus is in retreat.

While the numbers might still seem small compared to the peak of the pandemic this past winter, coronavirus cases are increasing once again.

The number of new infections Thursday was 12,809, up 15% from June 20, according to data compiled by The New York Times. Twenty-four states and the District of Columbia have seen increases over the past two weeks — 19 states are up 23% or more.

Health officials say the increases are largely a combination of two factors: the spreading delta variant and low vaccination rates in some states.

The delta variant, which is more transmissible than other strains of the virus, is now found in all 50 states. It accounts for about a quarter of all new infections, the Centers for Disease Control and Prevention said this week, and is expected to become the country’s dominant strain in the coming weeks. 

Studies have shown the vaccines currently authorized in the U.S. remain highly effective against the delta variant. But much of America remains hesitant to be vaccinated, and the problem of rising cases is mostly hitting states that are lagging in vaccinating their residents.

“Looking state by state and county by county, it is clear that communities where people remain unvaccinated are communities that remain vulnerable,” CDC Director Dr. Rochelle Walensky said Thursday.

Of the 24 states where cases are rising, 21 have fully vaccinated less than half their populations.

Nevada in particular is struggling: It has seen the nation’s biggest spike in cases over the past two weeks — up 114% — has the most per-capita infections and has the country’s second highest hospitalization rate. 

The Biden administration announced Thursday that the federal government is prepared to send COVID-19 surge response teams to help get more vaccines administered in areas with low inoculation rates, an offer that Nevada Gov. Steve Sisolak promptly took advantage of by requesting help. The Silver State currently has vaccinated 42% of its population.

“The COVID-19 vaccines are highly effective at reducing the severity of cases, hospitalization and deaths and we must continue to leverage resources at the federal, state and local level to increase access and confidence and get as many Nevadans protected from this deadly virus as possible,” Sisolak said in a statement.

Arkansas, which has the nation’s third lowest vaccination rate at 34%, has seen the second highest increase in infections over the past two weeks (up 81%), is tied for second in per-capita cases and has the third highest hospitalization rate.

"We are now going in the wrong direction yet again with COVID-19 infections here in the state of Arkansas," Dr. Cam Patterson, chancellor at the University of Arkansas for Medical Sciences, said during the state’s weekly virus briefing this week.

"With July Fourth holiday coming up and eventually kids going back to school, we have to be concerned that this would be a trend that could continue. And if it does, it would appear that we may be in the beginning of the third surge of COVID-19 here in the state of Arkansas,” he added.

In Texas, new infections have risen by 39% in two weeks. Among the eight counties with the most per-capita cases in the country, Texas accounts for six of them — Dimmit, Reeves, Hopkins, Harrison and Nacogdoches. 

Missouri is also struggling. It has seen cases climb by 55% in two weeks, has the nation’s highest per-capita hospitalizations and is tied for second in infection rate. Nine of the United States’ 20 worst hotspot counties are in the Show Me State.

Concerns about the delta variant prompted health officials in Los Angeles County, California, this week to “strongly” recommend that everyone — regardless of vaccination status — wear masks in indoor public places.

"We have enough risk and enough unvaccinated people for delta to pose a threat to our recovery, and masking up now could help prevent a resurgence in transmission," Health Director Barbara Ferrer said Thursday.

California, which has a 50% vaccination rate, has seen infections increase 28% in the past 14 days.

COVID-19 cases are rising as Americans head into the Fourth of July weekend and are more likely to attend public and private gatherings. Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and President Joe Biden’s chief medical adviser, issued a reminder Thursday that the CDC’s guidance saying it’s safe to be in most public settings without a mask only applies to the fully vaccinated.  

“You can still celebrate. … If you are vaccinated, you have a high degree of protection. If you are not, you should wear a mask, and you should think very seriously about getting vaccinated,” he said.