(CNN) There are signs that the United Kingdom could be heading into a fall Covid-19 wave, and experts say the United States may not be far behind.
A recent increase in Covid-19 cases in England doesn't seem to be driven by a new coronavirus variant, at least for now, although several are gaining strength in the US and across the pond.
"Generally, what happens in the UK is reflected about a month later in the US. I think this is what I've sort of been seeing," said Dr. Tim Spector, professor of genetic epidemiology at Kings College London.
Spector runs the Zoe Health Study, which uses an app to let people in the UK and US report their daily symptoms. If they start to feel bad, they take a home Covid-19 test and record those results. He says that about 500,000 people are currently logging their symptoms every day to help track trends in the pandemic.
Spector says the study, which has been running since the days of the first lockdown in England in 2020, has accurately captured the start of each wave, and its numbers run about one to two weeks ahead of official government statistics.
After seeing a downward trend for the past few weeks, the Zoe study saw a 30% increase in reported Covid-19 cases within the past week.
"Our current data is definitely showing this is the beginning of the next wave," Spector said.
On Friday, that increase was reflected in official UK government data too, although it was not as large as the increases reported by Zoe loggers.
Data from the National Health Service showed that after falling for nearly two months, the seven-day average of new cases in England and Wales rose 13% for the week ending September 17 over the week before. The seven-day average of hospitalizations was up 17% in the week ending September 19 compared with the week prior.
The data aligns with what models have predicted would happen in both the UK and the US.
"They predicted that we'd get a June to July peak and then there'd be a month where nothing happened in August and then it would flatten in in August and September and then start again in October. So it's exactly matching what the modelers have have been predicting," Spector said.
In the US, some models have predicted that Covid-19 cases will begin to rise again in October and continue to increase into the winter. Experts are hopeful that because most of the population now has some underlying immunity to the coronavirus, this wave would be less deadly than we've seen in previous winters.
It's not clear what's driving the increase in the UK or whether it will be sustained.
"These trends may continue for more than a week or two, or they may not," said Kevin McConway, emeritus professor of applied statistics at the Open University in Milton Keynes, England.
Broken down by age, he says, there are clear increases among adolescents who are around middle school age and younger adults, those 25 through 34.
"It wouldn't be surprising if there were some increase in infection as people come back from summer holidays and as the schools reopen," McConway said in a statement to the nonprofit Science Media Centre. "Even if it is, there's certainly no clear indication yet that it will continue."
He's not the only one who needs to see more data before calling this the start of a new wave.
"Question one is, how significant is that rise? Is it, for instance, the beginning of something, a new wave, or is this a temporary blip because of all of the getting together around the Queen's funeral and other events that have been going on?" said Dr. Peter Hotez, who co-directs the Center for Vaccine Development at Texas Children's Hospital in Houston.
A second important question will be whether the increase is being driven by a new variant.
"That's the worst possible situation. Because historically, when that situation occurs in the UK, it's reflected within a matter of weeks in the United States," Hotez said. "That was true of the Alpha wave; that was true of the Delta wave; that was true of Omicron and its subvariants."
That's where the US may catch a break this time around.
Instead of new variants, Christina Pagel, a professor of operational research at University College London, thinks cases are going up in the UK because of a combination of waning immunity and behavioral changes.
Many people in the UK are several months past their last Covid-19 booster or infection, and government statistics show that just 8% of adults 50 and older have gotten an Omicron-specific vaccine since the government started its fall vaccination campaign in September. School and work have fully resumed after the summer holidays, and people are spending more time indoors as the temperature drops.
Immunity is also waning in the United States, and Americans have also been slow to get boosted. Just 35% of those for whom a booster is recommended have had one, according to CDC data.
The updated boosters in the US are slightly different from the ones in the UK. The UK is using vaccines that have been updated to fight the original version of Omicron, which is not circulating anymore. US boosters have been updated to fight the BA.4 and BA.5 subvariants, which are currently causing infections both here and abroad. It's not clear whether the strain differences will have an effect on cases or disease severity.
There are a mix of new variants -- offshoots of BA.4 and BA.5 -- that are waiting in the wings. They represent just a small proportion of total cases, but several are growing against BA.5, which is still dominating transmission.
"It is very likely that these will accelerate current increases and cause a substantial wave in October" in the UK, Pagel said in an email to CNN.
Other experts agree with that assessment.
"There is talk about a bunch of lineages with concerning mutations, including BA.2.75, BQ.1.1, etc, but none of these are of high enough frequency in the UK right now to be driving the change in cases," Nathan Grubaugh, who studies the epidemiology of microbial diseases at the Yale School of Public Health, said in an email to CNN.
He says the mix of variants in the UK seems to be much the same as it is in the US, at least for now.
"We are seeing the increase in many respiratory viruses right now in the US, so it's not a stretch to think that a new COVID wave (or ripple) will be coming soon," he wrote.