It may be time for the U.S. to abandon the rigid confines of its vaccine distribution system and veer instead to more flexible and fluid rules that get more shots into more arms, say some leading health and science experts.

The urgent calls for a rapid increase in vaccinations come as the more contagious coronavirus variant first detected in the United Kingdom continues to spread across the country and fears grow that another, potentially more troublesome, variant discovered in South Africa threatens to reach American shores.

Mathematical modeler and physician Ronald Scott Braithwaite and his team at NYU School of Medicine have been helping the City of New York model out vaccine distribution scenarios. The city Wednesday announced it will soon start vaccinating 24/7, based on the CDC’s priority group schedule.

• Vaccine Distribution Has Begun. What Happens Next?

Braithwaite says distribution also needs to be kicked into high gear across the country. His modeling shows that New York City could achieve herd immunity with just 10 to 20 percent of its population vaccinated, combined with social distancing, masks and hand hygiene, meaning widespread community transmission of the virus would stop. But with the new, more contagious, variant from the UK in the mix, Braithwaite warns many more need to be vaccinated – quickly.

“Even with all the masks and the social distancing and everything, you would need to vaccinate 40 to 50 percent to prevent a second wave with the new intensely mutated variant. And that second wave would be a bad second wave, almost like the first wave,” said Braithwaite referring to March and April in New York City last year. “It's almost like a race against time. You've got to vaccinate enough to get to that 40 to 50 percent before that intensely mutated variant starts to spread and drive up the numbers.”

Braithwaite estimates it could take one to two months for the mutated strain to replace the existing strain. “Really, what we need to aspire to do is what Israel is doing, and that's vaccinating five percent of people every week. And that's a lot more than we've been doing, and it’s also quite a bit more than the one million a month. It's really two to four million a month. In my view, our targets need to be higher.”

Vaccine supply is not available to vaccinate widely, just yet. The greater issue, says Yale Medicine immunologist Akiko Iwasaki, Ph.D. is the rollout. “I think we need to correct the rollout and then increase the supply and then give it to as many people as possible,” said Iwasaki. 

One issue states have raised is the tension between strictly following priority group recommendations and the need to vaccinate as many people as possible. Experts say more guidance from the federal government is needed on who else can be vaccinated in the face of wasted doses. Dr. Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security says it is possible Wednesday's protests and rioting on Capitol Hill may divert even more attention from the rollout. 

“There are National Guard individuals that are being deployed and the National Guard have been also asked to help with vaccinations. So I think it's important that we as a country pour our resources into stopping this pandemic. And I think anything that distracts away from that is going to make it take longer to get the vaccine into the arms of people,” said Adalja.

Some experts, like Iwasaki, are among a growing chorus of experts urging the federal government to reconsider options like delaying second doses or even splitting first doses. The FDA shot those suggestions down Monday, holding firm to the plan to administer vaccine doses at a 21 day interval for the Pfizer-BioNTech vaccine and 28 days for Moderna. The agency says there simply isn’t “appropriate data” to support a change in the vaccine dosing schedule, which could put the public health at risk and undermine trust in the process.

“The more we let these variants circulate in the population, the more chances of the current vaccine not working properly. And I feel that it's kind of an emergency situation that we need to vaccinate as many people as possible right now in order to prevent further spread and further variants from becoming dominant in different populations,” said Iwasaki, pointing to concerns surrounding the South African variant that has the potential for so-called immune escape.

“The antibody and the spike protein are like lock and key, right? So it has to fit perfectly for the antibody to block that virus replication. Imagine the key changing its shape so that the lock doesn't fit anymore, that's kind of the situation that we might be facing with the South African variant, the spike is shifting its shape, so much so that the antibodies that are generated with the current vaccine may not fit very well anymore,” said Iwasaki.

A new study out by modelers at Stanford Medicine and the University of Toronto takes a look at the current rollout model – the “fixed strategy” which reserves 50 percent of each vaccine installment for second doses, and a more “flexible strategy” that would reserve just 10 percent of vaccine deliveries for second doses the first three weeks, reserve 90 percent for second doses the following three weeks, and 50 percent thereafter. The flexible strategy was found to prevent an additional 23 to 29 percent of COVID-19 cases compared to the current- fixed strategy.

When asked whether it would reconsider changing the dosing schedule, a spokesperson for the CDC referred to the FDA’s earlier statement. In a teleconference with press Wednesday, Dr. Nancy Messonnier, the CDC’s lead on COVID-19 vaccine efforts, says the country should start to see vaccine distribution pick up very soon.

“We launched this all during the holiday weeks. So, I think because of that, many jurisdictions had planned for a slightly measured initial weeks of rollout to get them comfortable with the vaccine and again, to account for it being the holiday week. Now that the holiday's over, I expect this program to continue to escalate and actually escalate really quickly,” said Messonnier. “I'm excited that more than four million people in the United States have already begun getting vaccinated and begun their protection against covid. And I think as we get more experience and as folks get comfortable with the vaccine, both in the public and those administering the vaccine, I think those numbers are really going to go up quickly.”