Brenda Goodman, CNN

As millions of Americans travel to gather with friends and family over the next few days, there's a good chance that COVID-19 will follow.

Experts expect that Thanksgiving gatherings will stir up social networks and give new coronavirus subvariants fresh pockets of vulnerable people to infect. As a result, cases and hospitalizations may tick up after the holiday, as they have for the past two years.

COVID-19 is not unique in this regard. Thanksgiving gatherings have the potential to amp up the spread of other viruses too, notably respiratory syncytial virus, or RSV, and influenza, which are both already at high levels for this time of year.

"We have seen, in some regions, RSV numbers starting to trend downward. Flu numbers are still on the rise. And we are concerned that after holiday gathering, lots of people coming together, that we may see increases in COVID-19 cases as well," Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, said Tuesday on CNN.

Respiratory illnessesFlu season hitting hard across half of US as respiratory illnesses increase
But things have been relatively quiet on the COVID-19 front. Experts say it may not stay that way for long.

"Covid positivity is going up," said Shishi Luo, associate director of bioinformatics and infectious disease at the genetic testing company Helix, which has been monitoring coronavirus variants. "It's increasing fastest among 18- to 24-year-olds" in the Helix sampling.

It's the first time test positivity in the Helix data has risen since July.

When test positivity increases, it means a greater proportion of COVID-19 tests are returning positive results, and it can be an indication that transmission is on the rise.

"We should expect more cases," Luo said. "Whether they're measured in how we measure cases right now, I don't know, but I think in general, you should see more people who are sick. I definitely am."

Increasing cases may not be picked up as quickly by official counts because people are mostly testing for COVID-19 at home and not reporting their results -- if they test at all.

The BQ subvariants of omicron have risen to dominate transmission in the US. BQ.1 and its offshoot BQ.1.1 are descendants of BA.5; they have five and six key mutations, respectively, in their spike proteins that help them evade immunity created by vaccines and infections. Because of these changes, they're growing more quickly than BA.5 did.

For the week ending Nov. 19, the CDC estimates that BQ.1 and BQ.1.1 were causing about half of all new COVID-19 cases in the U.S. But so far, they've risen to predominance without much impact.

COVID-19 cases, hospitalizations and deaths have remained flat for the past four weeks. But it's not gone: On average, more than 300 Americans die and 3,400 people are hospitalized each day with COVID-19, according to CDC data.

Nobody knows exactly what will happen with the BQ variants. Many experts say they feel hopeful that we won't see the big waves of winters past -- certainly nothing like the original omicron variant, with its jaw-dropping peak of nearly a million new daily infections.

There's reason for optimism on a number of fronts.

First, there's the experience of other countries like the UK, where BQ.1 has outcompeted its rivals to dominate transmission even as cases, hospitalizations and deaths have fallen. Something similar happened in France and Germany, notes Michael Osterholm, an infectious disease expert who directs the Center for Infectious Disease Research and Policy at the University of Minnesota.

"Cases went up in France and Germany just before the subvariants came in. Then the subvariants came in, and cases actually dropped," he said.

Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, thinks our behavior and our social contacts might be bigger determinants of whether cases will rise this go-round than whatever variant is in the lead.

He thinks it's likely that we'll see a rise in cases that may peak around the second week in January -- as it has in years past -- but that it won't have a big effect on hospitalizations and deaths.

Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, says that's probably because BQ.1's advantages are incremental, not drastic.

"It's probably got a bit more of a fitness advantage, so what we're seeing is gradual replacement without a massive change in the total number of COVID-19 cases," he said.

America lags in vaccination
All that's not to say that BQ.1 and BQ.1.1 won't have any impact. They've shown marked resistance to the antibodies that are available to protect and treat people who are vulnerable to severe COVID-19 infections. From that standpoint, there's good reason for people to be cautious if they have weakened immune systems or will be around someone who does.

But these subvariants will land at a time when population immunity is higher than ever, thanks to vaccines and infections. It's a very different setting than the virus encountered when omicron emerged a year ago, and that should also help dampen any coming wave, Pekosz says.

"With lots of people now being boosted and vaccinated and with people having some immunity from an omicron infection, it's also a very, very different sort of population landscape for a variant to emerge in," he said. "All the signs are, I think, the best part of the scenario in terms of not seeing these massive increases in cases."

If there's reason to worry about BQ in the U.S., it could be this: Americans aren't as well-vaccinated or boosted as other countries. CDC data shows that two-thirds of the population has completed the primary series of the COVID-19 vaccines, and only 11% of those who are eligible have gotten an updated bivalent booster. In the UK, 89% of the population over age 12 has completed their primary series, and 70% have been boosted.

New research indicates that a country's vaccination rate matters more than any other single factor when it comes to the effects of variants on a population.

Scientists at Los Alamos National Labs recently completed a study delving into what drove the effects of 13 dominant variants of coronavirus as they transitioned from one to another in 213 countries. The study includes data up to the end of September and was published as a preprint ahead of peer review.

Among 14 variables that influenced the speed and height of new COVID-19 waves, a population's vaccination rate was by far the most important.

The number of previous cases in a country, the percentage of people who wore masks, average income and the percentage of the population older than 65 ran a distant second, third, fourth and fifth, respectively.

How many other variants are in the mix when a new one rises is also an important factor, says senior study author Bette Korber, a laboratory fellow in the Theoretical Biology and Biophysics Group at Los Alamos.

She points to the Alpha variant, B.1.1.7, and how it behaved in the UK versus the U.S.

"When it came through England, it was just extremely fast, but it was much slower in the Americas," Korber said.

By the time Alpha reached the United States, we were evolving our own variants out of California and New York "that were very distinctive and had a competitive edge compared to what it had to come up against in England," Korber said, which probably slowed its roll here.

The CDC is tracking a soup of more than a dozen omicron subvariants that are causing cases in the U.S., and that variety may end up helping dampen any wave over the winter.

But Korber isn't making any predictions. She says it's just too difficult to know what's going to happen, pointing to Asia as the source of her uncertainty.

Asian countries have been contending with waves driven by the recombinant XBB, a subvariant that really hasn't had much of a presence in the U.S. The BQ variants arrived later, but she says they look impressive against XBB, which is also highly immune-evasive.

"BQ is really making a stand there," Korber said. "So I think it's not really possible to be certain yet" what could happen in the US.

"To me, it's a good time, when it's possible, to wear masks," she said. Masks protect the wearer as well as others around them. "And get the booster if you're eligible and it's the right moment for you," especially as we gather around the table to feast with our friends and family.

"It's a time to exercise a little additional caution to prevent that wave that we don't want to see happening, or at least make it a smaller bump," Korber said.