As the weather begins to cool, the United States is seeing an early rise in flu cases, and new Covid-19 subvariants are starting to grow in prevalence—leading health officials to express concerns about a "twindemic" of both viruses this winter.

Flu season is already ramping up in the US

Although flu transmission has been low throughout the pandemic, new data from CDC suggests that the United States is currently seeing an early start to this year's flu season, particularly in certain areas of the country.

According to Lynnette Brammer, a flu epidemiologist and team lead for domestic surveillance in CDC's influenza division, the flu positivity rate for the week ending Oct. 14 was 3% nationwide, but 5% in the South Central region and over 10% in the Southeast.

"Flu hospitalizations are going up," Brammer added. "And they're going up in the same places where flu positivity is going up and ILIs [influenza-like illnesses] are going up." Currently, most flu cases have been caused by influenza A, specifically the H3N2 strain, which has been known to cause more severe illness.

"Not everybody got flu vaccinated last year, and many people did not get the flu," said CDC Director Rochelle Walensky. "So that makes us ripe to have potentially a severe flu season."

So far, around 12 million flu vaccines have been administered in pharmacies and doctor's offices, which Walensky said is slightly less than the number of doses given at the same time last year—a fact that may be partially due to vaccine fatigue.

According to CDC guidance, everyone ages 6 months and older should get an annual flu shot to protect themselves from the virus. "We do want to get people protected before they have influenza in their own communities," Walensky said.

New Covid-19 subvariants raise concerns of a US surge

Although Covid-19 cases are still trending downwards in the United States, other areas of the world are currently seeing new surges in cases, largely due to highly transmissible and immunity-evading new subvariants.

The XBB subvariant, which descended from BA.2, is currently driving a significant increase in cases in Singapore, Bangladesh, and Hong Kong. In Singapore, new Covid-19 cases more than doubled in a single day, going from 4,700 to 11,700.

Similarly, several European countries, including the United Kingdom, Germany, France, and Belgium, are seeing new Covid-19 waves from the omicron subvariant BQ.1.1. So far, the United Kingdom has seen its average daily number of Covid-19 cases double over the past month, and the number of infections from BQ.1.1 specifically double every week.

In the United States, BA.5 continues to be the dominant subvariant, but the BQ.1 and BQ.1.1 subvariants are slowly gaining ground. According to CDC data, BQ.1 and BQ.1.1 each made up 5.7% of the total number of cases in the country last week.

"XBB and BQ.1.1 are 2 of the most important variants [to] watch right now," said Eric Topol, founder of Scripps Translational Institute, in a tweet.

According to Yunlong Richard Cao, an immunologist at Peking University in Beijing, both XBB and BQ.1.1, which have at least six advantageous mutations to their spike proteins, are some of the "most antibody-evasive" coronavirus variants ever tested. With antibody treatments less effective against these new subvariants, people who are at high risk of severe Covid-19 will be even more vulnerable.

While anti-body treatments may be less effective, Covid-19 vaccines are expected to stand a better chance against XBB and BQ.1.1. In particular, the new bivalent boosters from Pfizer-BioNTech and Moderna will likely help increase protection, especially when it comes to hospitalization and death, against the emerging subvariants since they target the newer BA.4 and BA.5 subvariants.

"Even with immune-evasive variants, vaccine protection against what matters most—severe disease—remains intact," said Amesh Adalja, a public health expert at the Johns Hopkins Center for Health Security.

However, booster uptake in the United States remain low, with only 5% of eligible Americans receiving the updated booster since it was first authorized in August. Currently, health officials are encouraging people to get boosted as soon as possible, especially as colder weather and the holidays approach.

"What happens in the weeks and months ahead will have a large impact on how the winter goes," said Ashish Jha, the White House Covid-19 response coordinator. "And really what happens in this winter is largely up to us, as the American people." (Shapero, The Hill, 10/14; Branswell, STAT, 10/17; Edwards, NBC News, 10/14; Romano, Yahoo! News, 10/13; Gans, The Hill, 10/14; Axe, Daily Beast, 10/16; Irfan, Vox, 10/13)