By the time the box office hit its stride, the delta variant appeared.

A slow and steady increase in box office revenue could be threatened by a new variant of the coronavirus.

Since March, the domestic box office has gained momentum, recently totaling up to $ 98.7 million in ticket sales over the past weekend, a pandemic record.

About 80% of cinemas are open to the public this weekend and restrictions on masks were relaxed for those who have been vaccinated against the coronavirus. As theaters continued to reopen and cases of Covid waned, studios were confident to release blockbuster footage.

Last weekend, Universal “F9” corresponds the highest opening weekend of any movie released during the pandemic with a $ 70 million haul, and helped bring the weekend’s total haul to $ 98.7 million, another industry record.

More than half of the American population has received at least one dose of the vaccine, leading to a sharp drop in the number of deaths from Covid, according to the Centers for Disease Control and Prevention. The number of new cases had declined considerably in recent weeks, and remains well below its peak. However, public health officials are monitoring the spread of the highly contagious delta variant in communities with low vaccination rates, which has resulted in a recent increase in cases.

The fear is that an increase in cases of delta variants could lead to a resurgence of health and safety restrictions not only nationally but internationally and deter moviegoers around the world from going to the movies.

Already, cities like Los Angeles have chosen to reverse mask guidelines, strongly recommending that unvaccinated and vaccinated people wear masks in public places like restaurants, stores and movie theaters. Movie chains told CNBC that theaters will continue to meet all local mandates.

“Every state is loosening up mask strategies, so if there is a push, many companies could be affected,” said Eric Handler, media and entertainment analyst at MKM Partners. “I hope the vaccines can help fight it enough so that it is not a major problem.”

CDC Director Rochelle Walensky said on Wednesday that the United States has a high vaccination rate and his highly effective vaccines allowed him to relax mask restrictions and social distancing measures. By comparison, much of the world is still unvaccinated, and current evidence suggests that vaccines developed outside the United States have been less effective.

“The global outlook also remains important, as studios and exhibitors closely monitor major international markets and how their various vaccine distributions are going in the second half of the year,” said Shawn Robbins, chief analyst at Boxoffice.com.

In some countries where the delta variant is gaining ground, new containment measures have been imposed.

The delta variant currently accounts for about 25% of new cases sequenced in the United States, and officials say they expect it to become the dominant strain in the country. In some countries, rates of delta variants can reach 50%, according to the CDC.

The US government is concerned about the pockets of the United States where vaccination rates are low. At the national level, approximately 1,000 counties in the United States have immunization coverage below 30%, Walensky said Thursday. These counties are mostly located in the Southeast and Midwest and are the most vulnerable to Covid infection, she said. The agency is already seeing increased disease rates in those counties due to the increased spread of the more transmissible delta variant.

“The film industry, like many others, has been considering variant scenarios similar to the current one as part of reopening plans for some time and nothing is taken for granted,” Robbins said. “At this time, however, there is no indication of an imminent impact on national cinemas as long as those vaccinated remain protected, as health officials have widely reported.”

– CNBC Rich Mendez contributed to this report.

Disclosure: Comcast is the parent company of NBCUniversal and CNBC. NBCUniversal is the distributor of “F9”.