Flu activity is still high, but it continues to drop in most regions, the US Centers for Disease Control and Prevention (CDC) said today in its weekly report, which covers data ending Dec 31.

Most markers declined, including the percentage of outpatient visits for flulike illness, which dropped from 6.1% to 5.4%. Also, the percentage of respiratory samples at clinical labs testing positive for influenza declined from 19.8% to 15.0%.

Thirty-nine states reported high (27) or very high (12) flu activity, another indicator of clinic visits for flulike illness. The numbers are down from 44 states reporting high (19) or very high (25) activity the previous week.

Testing at public health labs showed a bump in the proportion of 2009 H1N1 viruses, which increased from 16.9% to 29.8%.

Hospitalization, however, leveled off after dropping for 3 weeks in a row.

The CDC reported 13 more pediatric flu deaths, lifting the season's total to 74. The deaths occurred between the end of October and Dec 24. Of the 13 deaths, 12 were linked to influenza A and 1 was due to influenza B. Of the six subtyped influenza A viruses, five were H3N2 and one was 2009 H1N1.

A pair of new studies describe infection- and vaccine-conferred immunity against the SARS-CoV-2 Omicron variant, with one showing that vaccinated people have good protection against BA.5 subvariant reinfection for at least 8 months after infection, and the other finding that the vaccine is 46% and 70% effective against infection and hospitalization, respectively, in children.

Hybrid immunity 'quite stable'

Researchers in Portugal mined a national COVID-19 registry for data on more than 9.3 million vaccinated people 12 years or older from Jan 1 to Sep 14, 2022, a period of Omicron BA.1 and BA.2 predominance. The team compared previously infected people with their uninfected peers.

In Portugal, virtually all people had received a vaccine booster dose. The research was published yesterday in The Lancet Infectious Diseases.

The previously infected group initially was about 90% protected against reinfection, with protection falling to 70% after 5 months and then stabilizing at 65% at 8 months. Their relative risk, compared with that of COVID-naïve participants, was 0.24 initially and 0.32 at 3 months.

"These results show that hybrid immunity conferred by infection with previous subvariants of SARS-CoV-2 in vaccinated people is quite stable," co-senior author Luis Graca, MD, PhD, of the University of Lisbon said in an Instituto de Medicina Molecular news release.

Protection highest after booster

In the second study, published yesterday in the Journal of Infection, Chinese researchers conducted a meta-analysis of 14 observational studies on COVID-19 vaccine effectiveness (VE) in children and adolescents. The studies, published from Nov 1, 2021, to Nov 30, 2022, included nearly 3.8 million children.

Pooled VE against Omicron infection in children was 46.3%, with a VE of 18% after the first dose and 50.7% after the second. Among adolescents, VE against infection was 54.4%, with 39.1% VE after the first dose, 60.6% after the second, and 63.4% after a booster. VE against hospitalization in both children and adolescents was about 70%.

VE against symptomatic COVID-19 was 45.2% in children and 73.1% in adolescents, with 26.4% VE after the first dose, 59.7% after the second, and 71.1% after a booster.

Three African countries reported more polio cases this week, all involving vaccine-derived strains, according the latest weekly update from the Global Polio Eradication Initiative (GPEI).

In North Africa, Algeria reported one circulating vaccine-derived poliovirus type 2 (cVDPV2) case involving a patient from Ouargla, bringing its total for 2022 to two.

Meanwhile, the Central African Republic (CAR) reported one cVDPV2 case, its fourth for 2022. The patient is from Sanitary Region 6.

Elsewhere, the Democratic Republic of the Congo (DRC) reported 11 cVDPV2 cases in three provinces (Haut Katanga, Haut Lomami, and Tanganyika), boosting its 2022 total to 221. The DRC also reported two circulating vaccine-derived poliovirus type 1 (cVDPV1) cases, both in Haut Lomami province, lifting that total for 2022 to 51.