US flu activity keeps trending downward, according to the latest FluView report from the Centers for Disease Control and Prevention (CDC).

Flu cases are declining across most of the country, the CDC said, with influenza A viruses waning and influenza B viruses showing varying levels of activity. That trend follows the typical seasonal flu virus patterns. The proportion of tests that were positive for flu fell to 9.8%, down from 11.5% the previous week, and the proportion of outpatient visits for flu remained below the national baseline for the second straight week, falling from 2.8% to 2.6%.

For the season overall, influenza A viruses have been the most frequently reported. Of the influenza A viruses collected so far, 92.7% have belonged subclade K, which contains mutations that developed after this season’s flu vaccine strains were selected.

Weekly hospital admissions for flu also declined, dropping from 5,640 the previous week to 3,050 this week. But an additional four pediatric deaths were reported this week, bringing the total for the season to 127. Although the CDC has classified the current flu season as moderate for adults, for children it’s been a high severity season.

The CDC estimates there have been 30 million illnesses, 370,000 hospitalizations, and 23,000 deaths from flu so far this season.

RSV is elevated but has peaked

Overall, the amount of acute respiratory illness causing Americans to seek health care is low, the CDC said in its respiratory illness update. But respiratory syncytial virus (RSV) activity picked up later than normal this year and is currently at elevated levels, though it appears to have peaked in most regions, and the virus isn’t making people sicker than in previous seasons. Emergency department visits and hospitalizations for RSV are highest among children aged 4 and under.

COVID-19 levels are low across most of the country.

The CDC also noted that human metapneumovirus (HMPV) activity is rising across the country, which is typical for this time of year. Symptoms of HMPV include cough, fever, nasal congestion, and shortness of breath.

Infants born to mothers who received mRNA COVID vaccination during pregnancy had a 36% lower risk of COVID–related hospital contact in their first 6 months of life, according to a study published in Pediatrics. The protective effect did not extend to other types of infections.

In this cohort study that looked at all live-born infants in Norway from March 2021 to December 2023, researchers led by a team at the Department of Community Medicine and Global Health at the University of Oslo looked at health outcomes among newborns whose mothers were vaccinated during pregnancy and compared them with those whose mothers weren’t vaccinated while pregnant.

The analysis found that maternal vaccination was associated with a 36% reduced risk of hospital care for COVID in early infancy (adjusted hazard ratio, 0.64), a period when infants are not yet eligible for vaccination. Infants aged 6 months and younger have the highest rates of pediatric hospitalizations due to COVID infection. Protection waned from 52% in the first two months of age to 24% for three- to five-month-olds.

Vaccine protects against COVID, but not other infections

“[Maternal] vaccination protects the infant indirectly through protecting the mother, and directly by stimulating the production of maternal antibodies, including immunoglobulin G (IgG), which is actively transported across the placenta, and immunoglobulin A, which is secreted in breast milk,” note the authors. “Maternal antibodies confer immunity to the infant before they gradually wane and disappear.”

One aim of the study was to determine whether maternal COVID vaccination would help protect against other types of infection in infancy, but the findings showed no evidence that COVID vaccination reduced the risk of hospital contact for infections other than COVID. 

The analysis had some limitations, including potential differences in health care use between vaccinated and unvaccinated mothers. The study’s main strength is its large sample size, with researchers looking at all live births in Norway over a roughly 2.5-year period.