Covid EG.5.1 or "Eris": Time to worry?

What is EG.5.1?

Don’t worry about it.

But what is it?

There are many circulating strains of SARS-CoV-2; all are genomic subvariants of Omicron. EG.5.1 is one of these Omicron subvariants. You don’t need to worry about it.

Can I worry about it, though?

Well … EG.5.1 is the genomic subvariant of Omicron that is rising fastest as a proportion of viruses detected by genomic surveillance of wastewater. (Wastewater is the primary means of monitoring Covid today, because most infections go untested and undetected, hospitalizations and deaths lag infections by weeks, and their surveillance suffers from limitations in data collection and attribution of causality—i.e., dying “from Covid” vs. “with Covid”.)

This rise in wastewater suggests EG.5.1 is more transmissible than other circulating subvariants.

That sounds like something to worry about.

Not really. When Omicron first appeared in late 2021, it was more transmissible, yet far less lethal than Delta. It resulted in a massive number of infections, but a much smaller proportion were severe. (The severity of Omicron was also tempered by widespread natural immunity and vaccination.) The scale here is much smaller. There’s no evidence that a surge is coming, or that infections with EG.5.1 are any more severe than infections with other Omicron subvariants.

But they could be. They could be!

Yes, they could be. But they probably won’t, or not by much. They might even be less severe.

But aren’t hospitalizations going up?

There was a small increase in hospitalizations with Covid recently in the U.S. There’s no evidence this is related to the rise of EG.5.1. Smart people are watching this as closely as they can, given the limitations in our surveillance methods.

You’re acting like this is no big deal!

It’s probably not.

I like to worry sometimes.

I understand. I used to, too.

I think because if I worry about these things that are unlikely to be serious, I won’t have to face the bigger issues in my life.

Yeah. I totally get that.

And I don’t ever want to have to go through what happened in 2020 again.

Me either. It was terrible and I’m still deeply affected by it.

Is there a graph that makes this look more complicated and concerning than it is?

Yes:

EG.5.1 is represented by the progressively enlarging yellow-orange bars in the above graph from CDC.

OK, fine, but what about the “FLip” mutations? Aren’t those scary?

The previously dominant XBB.1.5 strain is shrinking as a proportion of cases as EG.5.1 rises. However, XBB.1.5 has in some areas of the world acquired new mutations that render it less susceptible to neutralizing antibodies.

These mutations were given the clever nickname FLip because they involve switching between F and L mutations in adjacent positions encoding the spike protein.

When both these mutations are present on XBB.1.5, the spike binds more avidly to ACE2, the cell receptor for the virus. This likely increases the transmissibility of SARS-CoV-2 bearing the FLip mutation set.

Although XBB.1.5 + FLip represents <2% of cases globally, its proportion of cases is rising rapidly in Brazil and Spain.

However, cases have generally been mild.

Will the new boosters target EG.5.1?

No, they target Omicron XBB.1.5, which was dominant in June when FDA issued guidance to Moderna and Pfizer on what boosters to produce. But EG.5.1 and XBB.1.5 share almost identical spike proteins, which are what the booster is based on.

But couldn’t that be something to worry ab—

The takeaway is that SARS-CoV-2 continues to mutate and evolve in response to pressures from natural immunity, vaccines, and possibly use of therapeutics like monoclonal antibodies.

Omicron remains the dominant genomic variant, with much lower severity of illness than in previous waves of Covid (in the context of rising global immunity from previous infection and vaccines).

A new Omicron subvariant EG.5.1 is increasing rapidly, suggesting it’s more transmissible. A small increase in U.S. Covid hospitalizations has been noted, but it hasn’t been linked to EG.5.1. Because EG.5.1 is Omicron, it will most likely cause a generally low severity of illness, like other Omicron subvariants.

A new set of mutations called FLip on the XBB.1.5 subvariant likely also increase transmissibility of the virus. In Brazil and Spain, FLip mutations are present in a rapidly increasing proportion of cases. However, these cases also appear mild so far.

Viruses naturally acquire mutations that make them more transmissible. This is inevitable and expected. Any significantly more transmissible subvariant will naturally, inevitably become a more prominent strain. As this process marches on, it deserves close surveillance, but it remains to be seen when or if any increase in illness will result.

I just realized. I haven’t spent time with my kids in a while. I think I’ll plan a family activity this weekend.

That sounds like a great idea.

Thanks for talking to me about all this.

You’re welcome. I hope you feel better.

You want to come? We might paint pottery or something.

I’ll be there. I don’t really have a choice—I’m one of the voices in your head.

Wait, what?