As the northern hemisphere plunges into its frosty winter, many European countries are reporting an uptick in cases.

Experts have warned the next few months will see the emergence of new strains, as vaccine immunity wanes and countries lift restrictions on their COVID-weary populations.

Already, three new variants - BQ.1, BQ.1.1 and XBB - are causing concern.

Anthony Fauci, US President Joe Biden's chief medical advisor, this weekend described the new strains as "pretty troublesome" and warned against "declaring victory" over the virus too soon.

"We've got to keep our eye out on these emerging variants," he said.

There some reasons to be optimistic, and a few to be concerned. Here's what you need to know.

Chipping away at protection.

These are the most contagious variants yet, as the virus mutates to become more infectious (and hopefully less deadly) over time.

Their ability to evade immunity from previous infections is also much stronger, studies suggest, making reinfections more likely.

But most worryingly, BQ.1.1 and XBB appear able to evade antibodies built up through some monoclonal therapies.

The arrival of monoclonal therapies was viewed as a major step in combating the virus, providing protection to immunocompromised people who cannot receive the vaccine. This may prove a spanner in the works.

It's worth noting, though, that these studies still need to go through the peer-review process, given how quickly moving these new variants are.

Spreading, and spreading fast.

BQ.1 and BQ.1.1 now account for 10 per cent of cases in the US, where BA.5 still makes up nearly 70 per cent of all infections. They're also outpacing previous strains in many European countries heading into winter.

XBB has quickly taken over in Singapore, where it was first detected in August.

Since then, we've seen cases numbers double in a single day last week in Singapore and the strain make up more than half of the country's cases. It's been detected as far as Australia and the US.

It's hard to tell.

Thankfully, there is no evidence currently that they cause more serious symptoms than previous strains. Some experts believe that could actually create an ideal scenario: a highly-contagious variants which send fewer people to hospital taking over.

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But the joker in the pack is long COVID.

We know that even people with initial mild symptoms can suffer serious, often deadly, long-term complications, ranging from debilitating fatigue to heart disease and diabetes.

It's too early to say whether these new subvariants are less likely to lead to long COVID.

Boosters are key.

The new variants appear adept at evading immunity from older-generation vaccines, which most Australians have received.

But as the strains have descended from BA.4 and BA.5 - both Omicron subvariants - Dr Fauci says Omicron-specific boosters will prove useful.

"There are almost certainly going to be some cross-protection that you can boost up," he said over the weekend.

Multi-strain vaccines also seem to provide some level of protection against XBB at least. Those jabs are still entering in the market, and aren't currently available to most Australians. The new strains show how important they're likely to be in the long-term.

Keeping up-to-date with boosters will keep your immune system as ready as possible, experts say.