(CNN) Colleges sending students home. Sports leagues canceling games and scrambling to deal with Covid-19 outbreaks. States returning to mask requirements.
The persistence of the Delta coronavirus variant, along with fear of the Omicron variant -- which vaccines are not as effective at stopping -- is complicating the US effort to emerge from the pandemic.
The main piece of advice from the government on Omicron: get a vaccine booster.
Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, laid out the evidence Wednesday during a virtual White House briefing. Two doses of the Covid-19 vaccine are not enough to stop the spread of the Omicron variant, but the addition of a booster shot can make the vaccine much more effective.
"The message remains clear: If you are unvaccinated, get vaccinated," said Fauci, who is the chief medical adviser to President Joe Biden. "And particularly in the arena of Omicron, if you are fully vaccinated, get your booster shot."
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Why isn't a booster required for "full vaccination?" Right now, the government is pushing hard for everyone who can get a booster to do so.
But it also has not changed the definition of "fully vaccinated" to include a booster. And for now, it's not going to.
Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention, did not offer up much of an explanation for why the government hasn't changed the definition.
"Certainly, as Dr. Fauci has demonstrated, and even our CDC data have also demonstrated, we are continuing to follow that science and it is literally evolving daily. And as that science evolves, we will continue to review the data and update our recommendations as necessary," Walensky said Wednesday.
Mixed messages. The unwillingness to update vaccine definitions is the latest example of governmental mixed messaging that could lead to some pandemic confusion.
The CDC has been back and forth on mask guidance as the pandemic has evolved and variants have emerged. Masks are recommended in indoor public settings by the CDC, but there's a new patchwork of rules about where they are required.
California enacted a new indoor mask requirement, but only for portions of the state. Also, you can still go unmasked to an indoor gym in San Francisco, for instance, where there's a vaccine requirement.
New York has a new temporary indoor mask requirement, but businesses that implement a vaccine requirement are exempted.
Those are states with relatively high vaccination numbers. In other states, where vaccinations lag, mask requirements have been outlawed.
An Omicron surge. The new variant only accounts for less than 3% of US cases, but the numbers could double every two days, Lori Tremmel Freeman, CEO of the National Association of County and City Health Officials, told CNN's Jacqueline Howard on Wednesday. That means Omicron could soon be the dominant US variant.
Even if this latest Covid-19 curveball, Omicron, does not for now appear to be as deadly as other variants, its quick spread is another aggravating setback in US efforts to pull out of the pandemic.
Headlines this week, like the outbreak among NFL teams and the new rules in California and New York, suggest outbreaks and restrictions are coming.
Omicron plus Delta. A less severe version of Covid-19 does not necessarily mean it isn't dangerous or deadly. Health officials say the addition of Omicron cases to Delta cases is what's concerning.
"It's the combination. It's kind of the perfect storm of public health impacts here with Delta already impacting many areas of the country and jurisdictions," Freeman said.
Hospitals already overwhelmed in parts of the US. Omicron has been detected in most US states and is causing concern among public health officials, but it is the Delta variant that still dominates in the US, and many hospitals are still overwhelmed.
"I have been practicing for 25 years in the emergency department, and every shift I am working these days is like the worst shift in my career," Dr. John Hick, an emergency physician at Hennepin Healthcare in Minnesota, said in a briefing. Other doctors described hostile patients making their treatment even more difficult.
Does the vaccine protect against Omicron? Yes and no. A study from South Africa released this week suggested that the two-dose Pfizer/BioNTech vaccine was not ideal at slowing the spread of the disease -- it was 33% effective at stopping infection.
But infection is not the whole story. The same study found vaccinated adults to be less likely to be hospitalized. It also found Omicron generally to be less likely to lead to hospitalization than the original strain of Covid-19.
CNN's Maggie Fox writes: "Government officials stressed that even if the variants can sneak by some of the protection offered by vaccines, vaccinated people tend to have milder disease or even no symptoms at all when infected -- so vaccines remain the No. 1 choice for protecting people. Masks, social distancing and other measures also work."
What works best against Omicron? Boosters. "Our booster vaccine regimens work against Omicron. At this point, there is no need for a variant-specific booster," said Fauci on Wednesday. "If you are unvaccinated, you are very vulnerable not only to the existing Delta surge we are experiencing but also to Omicron."
How many Americans have received boosters? A little more than 60% of the US population is fully vaccinated. Far fewer Americans -- about 1 in 6 -- has followed the latest guidance and received a booster.
The lack of booster uptake in the most vulnerable populations is perhaps more troubling. A little more than half of US nursing home residents have received a booster. CNN's Deidre McPhillips has a breakdown of the US population's vaccination status.
Anti-Covid-19 pill also shows promise. Pfizer has asked the government for emergency use authorization of a Covid-19 treatment -- the experimental antiviral pill Paxlovid -- which the drug company says cuts the risk of hospitalization or death by 89% if given to high-risk adults within days of their first symptoms. The Biden administration has already ordered enough to treat 10 million people.
Omicron proves what we already know. The coronavirus will adapt and confound efforts to contain it. Joseph Allen, director of the Healthy Buildings program at Harvard University's T.H. Chan School of Public Health, argues in The Washington Post that the government should get more nimble at responding to new developments -- and, for starters, update guidance on masks and boosters.
Is there any question, as Allen writes, the US needs a new Covid-19 playbook for the new year?