It's appearing in European sites already, and showing a pretty big growth advntage. Previous reports I've seen suggest the next covid wave in the US will peak around February, but the way this new variant is growing it might peak in late december or january.
JN.1 is a variant of BA.2.86 (aka Pirola), which was detected several months ago. It takes BA.2.86, which already demonstrated traits to evade immunity, and adds a couple key mutations: "L455S and P681R - ⬆️⬆️Immune evasive AND ⬆️⬆️fusogenic." - @RaffyFlynnArt (med science, phd researcher) - nitter - twitter
They think it might be an omicron-like event. What happens in sites in Europe where it's showing up will probably be an accurate preview of how it effects everywhere else.
Here's a chart of the growth advantage: nitter - twitter
Here's a prediction from JWeiland, whose predictions have been pretty accurate in the past nitter - twitter They do not think it will be an omicron-like event, but will be significant.
Dr. Eric Topal - twitter, who still takes covid seriously but isn't a covid doomer, warns that it's showing the most growth advantage we've seen in a long time.
The new vaccine shots are supposed to protect against BA.2.86, but we don't know how well it will work against the newest variant JN.1, and we don't know how protective a recent infection with a different variant will be. Probably better to be exposed while your blood is flooded with covid antibodies, with as little virus as possible, rather than not.
Edit, the new vaccine works against JN.1
We now report that administration of an updated monovalent mRNA vaccine (XBB.1.5 MV) to uninfected individuals boosted serum virus-neutralization antibodies significantly against not only XBB.1.5 (27.0-fold) and the currently dominant EG.5.1 (27.6-fold) but also key emergent viruses like HV.1, HK.3, JD.1.1, and JN.1 (13.3-to-27.4-fold). In individuals previously infected by an Omicron subvariant, serum neutralizing titers were boosted to highest levels (1,764-to-22,978) against all viral variants tested.
https://www.biorxiv.org/content/10.1101/2023.11.26.568730v1
And, since the "free market" dictated that the new covid vaccines should be an expensive hassle to get, with side effects often making people feel sick for a day or two, not many people have gotten them. 👍
Taking measures to reduce the amount of virus you are exposed to will certainly help if you end up getting infected. On top of the vaccine, using a HQ Mask in crowds, use anti-viral nasal sprays (including while you get sick, if you do get sick), and air filtration in places you have control over should all help.
All these measures will protect from other less severe illnesses as well, because covid didn't replace any of them, it's just adding to the burden and making them all worse. Abnormally high rates of all manner of severe respiratory infections in various places around the world for two or three years running is probably evidence of this.
As of now, asking people to try to avoid covid is like asking people to avoid cigarette smoke if people started smoking indoors in every public and private building, but that's where we are.
And no, avoiding a cold won't make your immune system weak.. When it comes to covid and other viruses the only way to "excercise" your immune system safely is to get vaccinated.
A covid infection, on the other hand, might actually disregulate your immune response - (twitter).
Here's a twitter thread explaining what it would look like if covid is causing immune disregulation. - (twitter)
Once again I am putting it on the record that I, a nobody with an internet connection and a small amount of free time, thinks that essentially turning the world into a gain-of-function experiment for a highly contagious endothelial disease that harms the immune system was a really bad idea.
On the other hand, the honorable and wise psychopaths who run our governments took a look at all the data and asked hard quesions like: "wtf does all this mean? so it kills old people, who cares? why are we shutting things down for that?" They have well-paid experts advising them and they seem to think it's fine, so who knows? 
There was a huge spike at the end of 2021 when omicron arrived. 2022 had a pretty high rates most of the year but we didn't see another huge spike, so hospitals weren't as crushed by it. 2023 was relatively low after the beginning of the year, and then we stopped tracking hospital covid cases as closely, and dropped mask requirements in medical facilities, and are basically pretending it's a solved problem.
If you look at https://biobot.io/data/ and view "total results" you can see clinical data disappear halfway into 2023, while wastewater shows a recent wave.
Jn.1 has from less than 1% of all sequences sampled in the US to about 8% in a few weeks, which is pretty alarming growth. It's so new the cdc isn't tracking it separately from the variant it mutated from.
Meanwhile parts of Canada are showing more covid in wastewater than last winter, and it's looking more like the appearance of omicron.