As the U.S. nears 30 million confirmed COVID-19 cases, and the arrival of spring brings warmer weather across some parts of the country, health experts are concerned about a fourth wave of COVID-19 in the U.S.

New infections are rising in some states, even as the vaccine rollout accelerates nationwide.

The spread of COVID-19 variants and 'pandemic fatigue' are fueling the rise, The Washington Post reports.

More states are lifting COVID-19 restrictions. Over the weekend Florida declared a state of emergency and imposed a curfew after large crowds of spring breakers gathered in Miami.

The White House is encouraging states to "slow down the relaxation" of COVID-19 guidelines. Although rates of new infections and hospitalizations are decreasing overall, the number of sick people remains high.

Among those warning of a new wave of infections is U.S. Centers for Disease Control and Prevention (CDC) Director Dr. Rochelle Walensky. She says the variants as the main cause for concern as restrictions are lifted.

"We must act now, and I am worried that if we don't take the right actions now, we will have another avoidable surge – just as we are seeing in Europe right now and just as we are so aggressively scaling up vaccination,” Walensky said during Monday’s White House COVID-19 briefing.

But the U.S. might soon have a fourth COVID-19 vaccine on the market. AstraZeneca reported their vaccine to be 79% effective toward the virus in a new study.

The company also said the study found no serious side effects to the vaccine, including the risk of blood clots which led many European countries to pause their use of the medicine.

AstraZeneca expects to file an application for approval with the Food and Drug Administration in the coming weeks.

About 1 in 4 people in the U.S. have gotten at least one COVID-19 shot, and about 1 in 8 are fully vaccinated, according to the CDC.

Check out The China Report, our new weekly newsletter. Subscribe here!