(CNN) Four days after President Joe Biden declared the darkest days of the pandemic over, America is facing a burst of challenges that indicate Covid-19 is far from releasing its grip.
Time and again -- as social distancing, families forced to stay apart and economic upheaval battered morale -- the nation has shown it's ready for the nightmare to end. But the virus doesn't work on human or political timetables. Now there are warning signs that troubling days are ahead, threatening to escalate the political tensions of a period that has torn at bitter ideological divides.
It all adds up to a serious problem for the White House, which has touted its competence in managing the vaccine rollout and handling the Covid crisis it inherited.
On Thursday, Pfizer reported that protection from its vaccine appeared to be waning over time and that it was developing a booster that should be taken between six and 12 months after recipients got their second doses, in order to restore full effectiveness. The US Centers for Disease Control and Prevention and the US Food and Drug Administration followed up that announcement with one of their own, in an effort to assure Americans that they don't need to get booster shots yet and that those agencies will make the decision on when or whether those shots are needed.
The good news is that the vaccine still has an extraordinarily high rate of preventing serious illness and death. So the miracle of Covid-19 vaccines remains intact, as there had long been expectations that boosters would be needed. But the latest development does suggest it will be imperative to extend a huge government inoculation effort into the future.
That will further complicate the task facing the White House at a moment when millions of skeptical Americans are balking at a first round of injections despite the success of the vaccine rollout.
"It is hard to imagine we are going to be able to immunize 200 to 300 million people every year to this," Dr Zeke Emanuel, a former health policy adviser to President Barack Obama, told CNN's Wolf Blitzer.
"That would be a huge challenge. We are already having difficulty immunizing people in the States just for the first round; imagine having to do it every year."
There is increasing data to show that vaccine holdouts are disproportionately in states that voted Republican in the last election, underscoring the difficulty the Democratic White House has in boosting vaccination rates. A cluster of hot spots, meanwhile, in the Southern and Southwestern US threaten to not only increase cases among unprotected people but also to act as breeding grounds for new variants that could compromise the effectiveness of existing vaccines.
In another sign of the pandemic's enduring threat, the quick-fire spread of the Delta variant of the coronavirus has thrown a blanket decline in cases into reverse, with 24 states now suffering upward trends. The more transmissible properties of the variant mean that it is even more dangerous to the unvaccinated than previous incarnations of the virus.
Given rising cases in the summer, experts worry that the colder fall and winter months could see a further surge in cases, deaths and overloading of already exhausted hospital staff. While a new national crisis remains unlikely, severe regional outbreaks could revive the need for shutdowns, masking and social distancing -- and bring all the political tensions that come with such measures.
"By the end of the summer, the beginning of the fall, some of those places with well-below-average vaccination rates are going to be in full surge mode," Dr. Jonathan Reiner, a professor of medicine at George Washington University, told CNN's John King on "Inside Politics." "Other parts of the United States are going to look like no more pandemic."
The idea that political motivations could be dictating vaccine reluctance is all the more tragic because of the stellar effectiveness of the vaccines.
CDC Director Dr. Rochelle Walensky said at a White House Covid-19 briefing Thursday that "99.5% of deaths from Covid-19 in the United States were in unvaccinated people."
"Those deaths were preventable with a simple, safe shot," Walensky said.
Perceptions that millions of Republican voters are risking what West Virginia GOP Gov. Jim Justice calls a "death lottery" were given fresh credence by a new report released on Thursday by the Kaiser Family Foundation, which showed a widening discrepancy in vaccination rates between counties that voted for Biden and ones that voted for Republican then-President Donald Trump last November.
In April, Trump country had an average vaccination rate of 20.6%, compared with 22.8% in Biden territory. By July, corresponding rates stood at 35% and 46.7%, a 9.5 percentage point jump in the gap.
The message of such data is clear: The nation's hopes of eradicating Covid-19 may increasingly rest on the willingness of Republicans to change their minds about the vaccines.
This group is the least likely to be convinced by Biden's appeals to take the shot and has an ingrained distrust of government. It is also more likely to be influenced by misinformation about the vaccine program that proliferates on conservative media and social media networks.
The White House has announced new approaches to reach those unwilling to get vaccinated, including a greater reliance on general practitioners and pediatricians to reach young people older than 12, who are eligible to be vaccinated. It has also sent rapid response teams into areas where the virus is particularly widespread and where vaccine reluctance is high.
In recent days, officials -- including Biden and the government's top infectious diseases specialist, Dr. Anthony Fauci -- have been on television pleading with people to get their shots.
"Please get vaccinated now. It works. It's free. And it's never been easier, and it's never been more important," Biden said on Tuesday.
"Do it now -- for yourself and the people you care about; for your neighborhood; for your country. It sounds corny, but it's a patriotic thing to do."
But some public health experts now think a more coercive approach might be needed -- even if the slightest suggestion of mandating vaccines would inflame conservative opinion. Right-wing pro-Trump Republicans like Reps. Marjorie Taylor Greene of Georgia and Lauren Boebert of Colorado have already this week compared Biden's vaccine teams to Nazis.
Dr. Leana Wen, a former Baltimore health commissioner, told CNN on Thursday that the administration should try changing its tone -- and start to stress that collective vaccination in personal and professional settings represents the best route to staying healthy.
"The federal government should be clear that vaccines are not just about the individual right now. There seems to be this messaging coming from the Biden administration that if you are vaccinated you are protected," she said, noting that such a line did not take into account people who remain immunocompromised or the possibility of breakthrough infections.
Such an adjustment might convince more businesses, schools and workplaces to put their own vaccine mandates in place and to encourage the wider effort to inoculate as many Americans as possible, Wen said.
Her argument gets to the most challenging aspect of this new phase of a crisis that, while far less severe than it once was, is also a long way from ending.
"Our problem at the moment is not, 'Can Pfizer produce a vaccine?' " Emanuel said. "The problem is, 'Will Americans take it?' "