The Labor Day holiday weekend is causing fears of a possible fourth wave of a spike of COVID-19 cases as more children unable to get vaccinated are ending up in the hospital.

From backyard barbecues, full flights and packed football stadiums for college games, Americans celebrating the unofficial end of summer could further threaten progress from COVID-19.

According to data from Johns Hopkins University, COVID cases during the Labor Day weekend rose 300% higher than the same weekend last year. The Transportation Security Administration reports that 3.5 million people traveled last weekend on Friday and Saturday. However, health officials warn that for the unvaccinated the Delta variant continues to pose a major threat.

In New York City, Mayor Bill de Blasio announced that 10.8 million doses of the COVID-19 vaccine have been administered. The seven-day average positive test results in the city was at 2.33% on Monday.

While some private and Catholic schools have already begun classes for the school year, New York City Public Schools are slated to open next Monday. The school year begins as a high number of children under 12 who are unable to be vaccinated are ending up in the hospital with COVID nationally.

According to the American Academy of Pediatrics (AAP), children represent over a quarter of new COVID-19 cases. As of Sept. 2, over 5 million children have tested positive for COVID-19 since the onset of the pandemic. About 252,000 cases were added the past week, the largest number of child cases in a week since the pandemic began.

“This year it will be especially important to keep our children healthy, as we’ve seen hospital beds and emergency services fill beyond capacity in communities where transmission of SARS-CoV-2 and other respiratory viruses remains high,” Dr. Flor Munoz of AAP said. “This means catching up on all immunizations, including the flu vaccine, and making sure children wash hands frequently, wear masks in school and during indoor group activities, and maintain physical distance from others.”

New York City school children are required to wear face masks in school. Teachers and school staff are required to get the COVID-19 vaccine and there will be biweekly COVID testing in schools. There is no option for remote learning.

“In many ways the health and safety protocols that the Department of Education is putting into place already go beyond the state guidance, perhaps most importantly with respect to vaccination and ensuring that all school staff are vaccinated, which is stronger than what’s elaborated in the state guidance,” said NYC Department of Health and Mental Hygiene Commissioner Dave Chokshi.

The World Health Organization is monitoring the Mu COVID-19 variant that originated from Colombia classifying it a “variant of interest.” Reports indicate that the Mu variant is in 49 U.S. states.

Chief Medical Advisor to the President Dr Anthony Fauci said this week that the variant has no immediate threat but “it might evade the protection from certain antibodies.” Health officials say more studies need to be done to know how powerful the variant is and its potential impact.

In the city, the Mu variant is making up less than 1% of positive COVID cases due to the Delta variant making up a majority of positive cases.

“We’re aware of this variant, we have been tracking it in New York City,” Chokshi said. “The prevalence has actually decreased significantly. Our public health guidance remains the same, particularly emphasizing the importance of vaccination against the Delta variant and for all of the other variants as well.”