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Summary

Introduction: the global influenza surveillance system is a network of laboratories and centers that monitor viral strains, including potential pandemic threats, vaccine candidates, and circulating isolates; this system now applied to respiratory syncytial virus (RSV) surveillance, tracks influenza’s seasonal patterns, which typically involve winter circulation in the Northern Hemisphere and summer circulation in the Southern Hemisphere; this year, the dominant influenza strain is AH1N1, with limited H3N2, and the prevalent B strain is B Victoria, marking a shift from the quadrivalent vaccines that included the now less common B Yamagata strain; evidence from the Centers for Disease Control and Prevention (CDC) showed that influenza accounted for 35% positivity tests and ≈8% of emergency department visits, with varying activity levels across the United States

Classification: the CDC classifies influenza season severity using influenza like illness (ILI), hospitalizations, and the percentage of deaths attributable to influenza; based on these markers, seasons are categorized as low, moderate, high, or very high severity

Current scenario: preliminary assessments as of January indicated a moderate season with 6.4% ILI, 17% weekly hospitalizations per 100,000, and a 1.6% peak mortality rate; current flu activity remains high, comparable to pre-pandemic levels, with some states experiencing very high ILI activity; the pediatric population, particularly children 0 to 4 yr of age and those 5 to 24 yr of age have been significantly affected; hospitalization rates increased, with the 18 to 49 yr age group showing the highest admission rates; pediatric mortality because of influenza is a serious concern; 80% of children who die from influenza are not fully vaccinated, and 50% have no underlying comorbidities

Vaccination: vaccination is recommended for everyone ≥6 mo of age; vaccination should continue throughout the season, older adults and those not in later stages of pregnancy may wait until September or October because of potential waning immunity; pregnant women in their third trimester should be prioritized for vaccination as early as August to protect themselves and their newborns; adult vaccination requires 1 dose, except for pregnant women in their third trimester, who may benefit from earlier vaccination; all current flu vaccines are trivalent, containing 2 A strains (H1N1 and H3N2) and 1 B strain (Victoria), because the Yamagata strain has not circulated since 2020; the key change this year is a new H3N2 component in both egg-based and cell-based vaccines; the intranasal flu vaccine (FluMist) has received approval from the US Food and Drug Administration (FDA) for home administration, but this will be implemented next year; standard dosing for children aged 6 mo to 8 yr — 2 doses for those never vaccinated or with unknown immunization status, and 1 dose for those who received 2 doses before July 2024; a grace period allows doses to be given ≤4 days before the minimum 4-wk interval between doses; the egg-based live attenuated influenza vaccine (LAIV) is approved for 2 to 49 yr of age with specific conditions; regardless of presentation, a 0.5 mL dose of inactivated vaccine is recommended for all ages

Guidelines: the American Association of Pediatrics recommends using a vaccine formulation not specifically approved for a child's age or administering a lower dose than recommended is considered valid, and revaccination is not necessary; for children requiring 2 doses, different vaccine brands can be used; the primary contraindication for inactivated and live influenza vaccines is a severe allergic reaction, particularly anaphylaxis; for the live, intranasal vaccine, additional contraindications include chronic airway disease, asthma, immunocompromised conditions, and cochlear implants; treating someone with antiviral medication for the flu can inactivate the live vaccine but does not influence the inactivated vaccine; the CDC’s Morbidity and Mortality Weekly Report (MMWR) clarifies contraindications based on vaccine type; a reaction to a cell-based vaccine contraindicates further use of cell-based vaccines only; a reaction to a recombinant vaccine contraindicates further use of recombinant vaccines

High risk populations: include young children, older adults, those with chronic diseases, immunocompromised individuals, pregnant people, aspirin users, nursing home residents, American Indian and Alaska native persons, and obese individuals

Solid organ transplant recipients: high dose vaccines may be beneficial for those aged 18 to 64 yr of age because of to their decreased immune response to standard vaccines

Immunocompromised pediatric patients: for those receiving chemotherapy, vaccination is advised ≥2 wk after treatment; for children on anti-B-cell therapies, vaccination is ideally given 2 to 4 wk before starting therapy, or 6 mo after if they are already on treatment, because of B cells’ role in antibody production; live vaccines are contraindicated for patients on CAR-T cell therapy; non-live vaccines can be given 6 mo after treatment; bone marrow transplant recipients can receive the vaccine 4 to 6 mo after transplant; Schuster et al (2024) showed that high dose influenza vaccines may increase antibody titers in pediatric bone marrow transplant recipients; this is an off label use and requires consultation with infectious disease specialists; solid organ transplant patients should receive the vaccine 3 mo after transplant, or 1 mo later if during flu season because of immunosuppression; household contacts of immunocompromised patients should also be vaccinated

Vaccine effectiveness (VE): for children <18 yr of age, outpatient VE ranged from 32% to 60%, while inpatient VE was between 63% and 78% for any influenza strain; the VE against the predominant AH1N1 strain was strong, ranging from 50% to 72%; VE against H3N2 was lower, especially in outpatient settings, and was ≈55% for inpatient cases; adult VE showed similar trends, with outpatient effectiveness ranging from 30% to 54% and inpatient from 40% to 55%; AH1N1 protection was better than H3N2; influenza vaccine coverage continues to decline, particularly since the pandemic; overall coverage is ≈55%, with young children having better coverage at 64%; adult coverage is <50%, with those ≥65 yr of age showing the highest coverage (≈70%); only 1 in 4 mothers receive both flu and Tdap vaccines

Antiviral treatment recommendations: treatment is advised for hospitalized patients, those with severe disease, and high risk individuals; it can be considered for any confirmed influenza case ≤48 hr of symptom onset; oseltamivir, peramivir, zanamivir, and baloxavir are approved antiviral drugs; oral agents — baloxavir is a single dose option for older children and adults; oseltamivir requires 5 days of treatment, and may be used in younger children; oseltamivir is preferred for both treatment and prophylaxis because of extensive experience, cost effectiveness, and efficacy against influenza A and B; treatment should begin based on clinical suspicion, not solely on positive test results; prophylaxis is not a substitute for vaccination and is typically reserved for high risk individuals

Key points: the CDC is monitoring the avian influenza AH5N1, which has been detected in cattle across multiple states; there is no evidence of sustained human to human transmission; the risk for a pandemic is currently considered low; testing for AH5N1 is recommended for hospitalized patients with severe influenza and potential exposure to cattle or birds