In December, then President-elect Joe Biden pledged to vaccinate 100 million people within his first 100 days in office, a goal aimed at speeding up a process that was off to a sluggish start. On Monday, he discussed raising that goal to 150 million doses in 100 days.

Hitting that number certainly presents its share of challenges – and some experts believe that it isn’t ambitious enough. Some public health experts, in fact, are pushing for the U.S. to upgrade its goal to two million vaccine shots per day, and possibly even three million.

“There are huge barriers that stand in the way of our country reaching the kind of numbers that we need to,” said Dr. Leana Wen, an emergency physician and public health professor at George Washington University. “I think we need to aim a lot higher in terms of the rate of vaccinations.”

At the start of 2021, the U.S. was administering fewer than 500,000 vaccinations per day. That number has slowly climbed since then – by January 26, the U.S. was averaging 1.1 million doses per day, according to data from the Centers for Disease Control and Prevention (CDC).

So even though the current goal of one million vaccinations a day has already been reached, it’s not enough. At the 100-million-per-day pace, only about 20% of the U.S. population will be vaccinated by April.

“The bar that’s set of 100 million vaccinations in 100 days is a disappointingly low bar,” Wen said. “At this point, we need bold solutions to stop this raging pandemic. If you’re at a rate of one million vaccinations a day, we won’t get to herd immunity until 2022.”

Taking longer to vaccinate more Americans poses critical problems. For one, the longer the pandemic continues, the more people will die from the virus. Currently, more than 3,000 Americans are dying from COVID-19 every day.

A slower mass vaccination rollout also means a higher risk of mutated versions of the virus, which could prove more dangerous. “Even with the revised goal of 150 million vaccinations in 100 days, we’re still looking at the end of this year,” Wen explained. “We’re in a race against time, especially with the variants.”

But expanding the current goal to more than one million doses per day may be difficult due to a limited vaccine supply and a lack of data to track it, CDC director Rochelle Walensky has noted.

There has also been a slowdown in distribution of the vaccines, largely due to a lack of communication and coordination between the federal government and state and local health departments. This plagued the Trump administration’s initial efforts to deploy the vaccines, which fell well short of the original goal of 20 million doses before the end of 2020.

Biden’s new plan, by contrast, involves large-scale vaccination efforts, including a steep increase in the number of vaccination sites across the country. Previously, Wen noted, “We had to rely on hospitals, nursing homes and select pharmacies [to administer the vaccines]. Now the plan is to have community sites and health centers to set up mass vaccination sites in particularly hard-hit communities.”

This will help speed up the process, but states continue to lag in administering the doses they’ve received. According to the CDC, while more than 44 million doses of the vaccine have been distributed across the U.S., states have only administered 23 million thus far.

Lack of funding for these mass vaccination sites could potentially prove to be a longer-term issue, especially since states are largely responsible for picking up the tab for vaccination programs, training, staffing and testing. “State and local health departments were substantially underfunded before the pandemic hit,” Wen said. “This has only gotten worse, because the same entities that have had to set up testing, contact tracing and public education have also had to set up mass vaccination programs. They absolutely need more funding.”

Passed at the very end of 2020, the most recent COVID stimulus bill includes some $8.4 billion for vaccination efforts, with $4.5 billion expected to go to states. Whether that funding will be enough to maintain mass vaccination efforts, however, isn’t certain.

A final issue has been a lack of consensus on guidelines for vaccine administration, whether around prioritizing certain age groups or determining whether states should be given more doses depending on their distribution rates.

“The federal government absolutely needs to be doing more to assist states, not in making things even more inflexible,” Wen stressed. “What they should be doing is supporting states and trying to streamline processes.”

In short, states shouldn’t be required to reinvent the wheel, and local health departments shouldn’t be creating their own training manuals. “It would also be helpful if the federal government were to come out with a video and paperwork that needs to be signed by everyone ahead of getting vaccination,” Wen added.