By Christina Maxouris, CNN

(CNN) — Lowering Covid-19 cases across the US isn’t only critical to helping prevent another surge fueled by variants — it also makes vaccines more likely to continue working effectively against mutations, one expert says.

“The best thing we could possibly do to improve the chances that the vaccine will continue working as we’re hoping it will, is to reduce cases as much as possible without having those reductions occur as a result of vaccine-derived immunity,” Dr. Michael Mina, an epidemiologist and immunologist at the Harvard T.H. Chan School of Public Health, said during a Tuesday event.

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The US on Tuesday reported more than 59,500 new Covid-19 cases — a far cry from the six-figure case totals it was reporting just a month ago. But experts warn that these infection numbers are still high and could see another uptick if Americans let down their guard.

This is why measures like masks and social distancing continue to play a key role, because if the virus continues circulating at high levels, it will likely more often come into contact with, and try to infect, people who have been vaccinated, Mina said.

“The more opportunities we give the virus to come in contact with somebody who is immune, the more opportunities there are for the virus to find a way around that level of immunity and those antibodies,” Mina added.

Scientists have already identified several variants circulating in the US and are particularly worried about the B.1.1.7 strain — a highly contagious variant first detected in the UK. One study recently found cases of the variant are rapidly increasing across the US and significant community transmission may already be occurring.

More than 1,270 cases of the variant have been reported across 41 states and Washington, DC, according to data from the Centers for Disease Control and Prevention. Roughly a third of those cases are in Florida, the CDC data shows.

But the agency has said that likely doesn’t represent the total number of cases nationwide and has recently worked to ramp up its genome sequencing efforts in order to identify variants.

On Tuesday, Quest Diagnostics announced it was also sequencing tests to support the CDC’s efforts to track mutations, adding that the company had already doubled the amount of genomic sequencing it started performing last month.

Fauci shifts vaccine timeline for Americans public

The US is also facing challenges on the vaccine front.

More than 39 million people have so far received at least the first dose of a Covid-19 vaccine, according to CDC data. About 15 million people — roughly 4.5% of the US population — have been fully vaccinated, the data shows.

Dr. Anthony Fauci told CNN on Tuesday it could now be mid to late May or early June before vaccines are available to the general population — shifting the timeline from previous estimates which placed widespread vaccine availability around the end of April.

“That was predicated on (the Johnson & Johnson vaccine) having considerably more doses than now we know they’re going to have,” Fauci said. “So, that timeline will probably be prolonged maybe into mid to late May and early June.”

CNN has learned that if it gets the green light from the Food and Drug Administration, the rollout of the Johnson & Johnson vaccine will be slower than officials initially expected.

The administration expects single-digit millions of doses for the vaccine if it’s authorized for emergency use. However, due to a miscommunication over the production timeline, government officials thought that the number would ramp up to between 20 or 30 million doses by April. But now, they’re expecting fewer than 20 million doses in April, an administration official told CNN.

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Separately, President Joe Biden promised on Tuesday that 600 million Covid-19 vaccine doses would be available to the country by the end of July, “enough to vaccinate every single American.”

The President’s promise follows his announcement earlier this month that the US will have enough vaccines for 300 million Americans by the end of July, with the help of additional vaccine purchases the government made from Pfizer and Moderna.

“What’s going to happen is it’s going to continue to increase as we move along. We will have reached 400 million doses by the end of May and 600 million by the end of July,” Biden said at a CNN town hall event.

More vaccine challenges: inequity, weather and short supply

Among the other difficulties the US is working to overcome is inequity in vaccine distribution, frigid weather and a short supply.

New York City officials said Tuesday they were working to combat Covid-19 vaccine disparities, building off of efforts that have focused on outreach to some of the region’s hardest-hit communities.

But newly released data shows “the scale of the challenge in front of us,” said Dr. Torian Easterling, first deputy commissioner and chief equity officer for the city’s health department.

“There’s much smaller proportion of vaccines going to Black and Brown New Yorkers. We see these geographic disparities bearing out as well,” Easterling said, adding that the South Bronx, parts of Central Queens and Central Brooklyn, lag behind in vaccinations.

Meanwhile, the fierce winter storms that have unleashed frigid and dangerous weather conditions have now slowed vaccinations across the country.

Ohio Gov. Mike DeWine said Tuesday vaccine shipments being delivered to the state’s providers directly from Pfizer and Moderna could be delayed by one or two days because of the weather. In Georgia, health officials said they were also expecting delays in vaccine arrivals, adding many providers and health departments were being forced to reschedule vaccine appointments.

For some parts of the country, supply problems are slowing the shots.

The Beaumont Health system in Michigan canceled more than 1,880 second-dose appointments that were scheduled for Thursday after it learned of an “unexpected and significant reduction” in Pfizer vaccine allocation from the state, system officials said in a news release.

“The health system is working to automatically reschedule all canceled appointments to one week later at the same time and on the same day of the week, as long as the state supplies enough vaccine,” the release said. “Beaumont is also seeking more clarity from the state on these much needed second doses.”

Michigan Department of Health and Human Services spokesperson Lynn Sutfin said the state was working with Beaumont Health for the past week to reconcile its second-dose shortages, adding it was “unfortunate that they chose to cancel second-doses appointments while we were continuing to work with them on this issue.”

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