WASHINGTON (REUTERS) - The US Centres for Disease Control and Prevention (CDC) is stepping up its efforts to track coronavirus mutations and keep vaccines and treatments effective against new variants until collective immunity is achieved, the agency chief said on Sunday (Jan 24).

Dr Rochelle Walensky spoke about the rapidly evolving virus during a Fox News Sunday interview as the number of Americans known to have been infected surpassed 25 million, with more than 417,000 dead, just over a year after the first US case was documented.

Dr Walensky, who took over as CDC director last Wednesday, the day President Joe Biden was sworn in, also said the greatest immediate culprit for sluggish vaccine distribution was a supply crunch worsened by inventory confusion inherited from the Trump administration.

"The fact that we don't know today, five days into this administration, and weeks into planning, how much vaccine we have just gives you a sense of the challenges we've been left with," she told Fox News Sunday.

Mr Biden's transition team was largely excluded from the government's vaccine rollout deliberations for weeks after his election as then-President Donald Trump refused to concede defeat and allow the incoming administration access to information needed to prepare to govern.

Mr Ron Klain, Mr Biden's chief of staff, said in a separate interview on NBC's "Meet the Press," that a plan for distributing the vaccine, particularly beyond nursing homes and hospitals, "did not really exist when we came into the White House."

Dr Walensky said she was confident the government would soon resolve supply questions, and go on to dramatically expand vaccine production and distribution by late March.

Uncertainty over immediate supplies, however, will hinder efforts at the state and local levels to plan ahead for how many vaccination sites, personnel and appointments to set up in the meantime, exacerbating shortages in the short term, she added.

Race against variants

Vaccination has become ever more urgent with the recent emergence of several coronavirus variants believed to be more transmissible, and in the case of one strain first detected in Britain, possibly more lethal.

"We are now scaling up both our surveillance of these and our study of these," Dr Walensky said, adding that the CDC was collaborating with the National Institutes of Health, the Food and Drug Administration and even the Pentagon.

The object, she said, is to monitor "the impact of these variants on vaccines, as well as on our therapeutics," as the virus continues to mutate while it spreads.

Until vaccines can provide "herd" immunity in the population, mask-wearing and social distancing remain vital to "decrease the amount of virus that is circulating, and therefore, decrease the amount of variants that are out there," Dr Walensky said.

Although British officials on Friday warned that the so-called UK variant of the coronavirus, already detected in at least 20 US states, was associated with a higher level of mortality, scientists have said existing vaccines still appeared to be effective against it.

They worry, however, that a more contagious South African variant may reduce the efficacy of current vaccines and shows resistance to three antibody therapeutics developed for treating Covid-19 patients.

Similarities between the South African variant and another identified in Brazil suggest the Brazilian variety may likewise resist antibody treatment.

"We're in a race against these variants," said Dr Vivek Murthy, nominated by Mr Biden to become the next US surgeon general, on ABC's "This Week" programme on Sunday.

Dr Anthony Fauci, the nation's leading infectious disease specialist, said in late December he was optimistic the United States could achieve enough collective immunity to Covid to regain "some semblance of normality" by the fall of 2021.

But Dr Murthy said getting to herd immunity before a new school year begins in September was "an ambitious goal."

Nevertheless, Dr Murthy suggested the government may exceed Mr Biden's objective of administering 100 million vaccinations in the first 100 days of his presidency, telling ABC News, "that's a floor; it's not a ceiling."

Dr Fauci, appearing separately on CBS News' "Face the Nation,"said the 100-million-shot goal includes those who may have received both injections of the two-dose vaccines and those who only got the first.

About 21.8 million Americans, or about 6.5 per cent of the population, have received at least one dose of vaccine to date, of the 41.4 million doses shipped, CDC data showed on Sunday.