(NewsNation Now) — President Joe Biden kicked off a more urgent campaign for Americans to get COVID-19 booster shots Thursday as he unveiled his winter plans for combating coronavirus and concerns about the omicron virus variant.

The plan includes a requirement for private insurers to cover the cost of at-home COVID-19 tests and a tightening of testing requirements for people entering the U.S. regardless of their vaccination status. But as some other nations close their borders or reimpose lockdowns, officials said Biden was not moving to impose additional restrictions beyond his recommendation that Americans wear masks indoors in public settings.

Biden said Wednesday that the forthcoming strategy, to be unveiled during a speech at the National Institutes of Health, would fight the virus “not with shutdowns or lockdowns but with more widespread vaccinations, boosters, testing and more.”

The Biden administration has come to view widespread adoption of booster shots as its most effective tool for combating COVID-19 this winter. Medical experts say boosters provide enhanced and more enduring protection against COVID-19, including new variants.

About 100 million Americans are eligible for boosters under current U.S. policy, with more becoming eligible every day. Convincing those who have already been vaccinated to get another dose, officials believe, will be far easier than vaccinating the roughly 43 million adult Americans who haven’t gotten a shot despite widespread public pressure campaigns to roll up their sleeves.

The administration will ramp up messaging to encourage adults, particularly seniors, to get booster doses at local pharmacies, including a town hall campaign through AARP.

The administration’s upcoming rule to require private insurers to cover at-home testing is still being drafted, and many details remain to be worked out, including under what criteria they will be reimbursable, officials said.

Those insured by Medicare and Medicaid would not be eligible, but the White House said as many as 150 million people with private insurance would see easier and cheaper access to the at-home tests. The administration said it is making 50 million COVID-19 tests free for older people and other vulnerable groups for pickup at senior centers and community sites.

Much remains unknown about omicron, including whether it is more contagious, whether it makes people more seriously ill and whether it can thwart the vaccines.

The campaign comes as scientists race to understand what to make of omicron, which went from newly discovered to the World Health Organization’s variant of concern list in a matter of days.

A person in California became the first in the United States to have an identified case of the variant, administration officials confirmed Wednesday. The nation’s top infectious disease expert, Dr. Anthony Fauci, told reporters the person was a traveler who returned from South Africa on Nov. 22 and tested positive Monday. Fauci said the person was vaccinated but had not received a booster shot and was experiencing “mild symptoms.”

It’s not clear where the new variant first appeared, but scientists in South Africa alerted the World Health Organization, and it has now been seen in travelers arriving in several countries, from Australia to Israel to the Netherlands.

“We’ve seen a sharp increase in cases for the past 10 days. So far, they have mostly been very mild cases, with patients having flu-like symptoms: dry coughs, fever, night sweats, a lot of body pains,” said Dr. Unben Pillay, a general practitioner in Gauteng province, where 81% of the new cases have been reported.

Adding to the confusion, the Netherlands announced Tuesday that the omicron variant was already in the country when South Africa alerted WHO about it last week. The Netherlands’ RIVM health institute found omicron in samples dating from Nov. 19 and 23.

The CEOs of Pfizer and Moderna, which make the highly effective mRNA vaccines used in the U.S., have both said they expect their formulas will need to be tweaked to maintain their current level of protection against omicron.

Still, they expect their vaccines will offer protection from severe disease. Firmer results from laboratory tests to determine the vaccines’ effectiveness is expected later this month.

Internationally, omicron is fueling a resurgence of strict pandemic mitigations. Japan is blocking any newly booked travel into the country through the end of the year.

Greece will fine residents older than 60 who don’t get vaccinated. European Union Commission President Ursula von der Leyen said EU nations should consider making vaccinations mandatory. And German Chancellor-designate Olaf Scholz said he will back a proposal that requires everyone to get vaccinated.