As fresh hope is injected into the fight against COVID-19 with the arrival of a new vaccine, there are concerning reports of a new strain causing cases to skyrocket in London.

First shipments of Moderna's coronavirus vaccine are driving optimism in the United States. However, British officials are sounding the alarm about a new strain of the virus which could be up to 70% more contagious than the original.

U.K. Prime Minister Boris Johnson is sending parts of the country, including London, back into full lockdown because of the new strain.

Several mainland European countries are suspending flights from Great Britain to keep the mutation away from their borders.

On a conference call with members of the media on Sunday, Gov. Andrew Cuomo urged the U.S. federal government to do the same. He says six flights a day come from the U.K. to New York.

"To me, this is reprehensible, because this is what happened in the spring," Cuomo said. "Other European countries have done a ban, we haven't. And today that variant is getting on a plane and landing at JFK."

Cuomo also issued a letter to President Donald Trump this weekend urging him to "use his influence and force Congress to ensure the next COVID-19 stimulus package includes real financial relief for American families, states and localities." Read Cuomo's letter to the president here.

Some U.S. doctors say they are not overly concerned about the new strain at this point. Dr. Bettie Steinberg with Northwell Health says viral mutations are common.

Experts also say it's likely that the new strain is not any more deadly than the original, even if it appears to be more contagious. It's believed that the vaccine, which is currently being distributed in Great Britain and in the United States, will be effective against this mutation.

Still, Dr. Steinberg says it's possible that the new strain could make it to the U.S. even with lockdown in place.

"If that strain has a slightly better advantage at replicating or infecting or whatever it is, it will spread," he says. "The original virus has changed many times."