National Nurses United (NNU), the largest labor union and professional association for registered nurses in the U.S., is asking the Centers for Disease Control and Prevention to bring back recommendations for universal masking regardless of vaccination status. The NNU wrote a letter to CDC Director Rochelle Walensky urging the agency to update its guidance as COVID-19 cases continue to soar across the country.

“The pandemic is far from over,” wrote Bonnie Castillo, the executive director of NNU. “The United States once again stands on the precipice of rising cases. NNU strongly urges that the CDC strengthen and improve its guidance to protect nurses, our patients, and the public.”

According to The Hill, the union cited the World Health Organization’s stance that vaccinated people should continue to wear masks in public, particularly as the highly transmissible Delta variant gains strength around the world.

While healthcare officials point out that the WHO recommendation is more appropriate for countries with lower vaccination rates than the U.S., Castillo says that while COVID-19 vaccines are effective in reducing severe illness, they are not 100% effective, and the emergence of more dangerous variants reduces their efficacy even further.

An Israeli study found that the Pfizer vaccine was only 64% effective in preventing COVID-19 infections due to the Delta variant although it did prevent severe illness by 90%, according to Axios. It was this study data that Pfizer presented to government officials earlier this week to seek authorization for a third or booster shot.

The CDC stands by its guidance that fully vaccinated people may go maskless in virtually all settings, but the nurses’ union says that the agency is putting frontline workers, nurses and others in danger. In the letter, Castillo points out that even those individuals who are vaccinated can spread the virus to others, including children who are not eligible for the shot.

Some countries have taken it upon themselves to make their own recommendations. Los Angeles County and St. Louis County have recommended that residents don masks indoors.

In 46 states, the rate of new cases has risen by at least 10% since last week, according to data from Johns Hopkins University, says CNN. In 31 states, the new cases this past week are more than 50% higher than the previous week. The majority of new COVID-19 cases have been in unvaccinated people.

In Mississippi, seven children are in intensive care due to the surge, according to the state health officer, Dr. Thomas Dobbs.

“Pretty much ALL cases in MS are Delta variant right now,” Dobbs tweeted Monday, adding that most of the hospitalizations and deaths were among unvaccinated individuals. He also said that 7% of deaths occurred in fully vaccinated patients, a fact he called “worrisome.”

According to TIME, Dr. Muntu Davis, health officer for Los Angeles Country said that the decision to recommend masks in public is based on the continued uncertainty about how much vaccine immunity protects against the Delta variant.

He said that even if there is a small chance vaccinated people can become infected by the variant, wearing masks in public is “prudent.”

Dr. Kirsten Lyke, associate professor in the Center for Vaccine Development at the University of Maryland, believes that mask recommendations are totally in line with the current COVID-19 profile.

“I personally err on the side of caution and wear a mask indoors and out even though I am fully vaccinated,” she said. “I just think we’ve been surprised enough by COVID-19 that I’m not sure we fully understand the Delta variant, and the degree to which it is transmissible.”