# Association of viral loads of influenza A (H3N2) with age and care setting on presentation—a prospective study during the 2022-2023 influenza season in Spain

## TL;DR

This study found that infants, children, and the elderly have higher influenza A (H3N2) viral loads, and outpatients have higher viral loads than hospitalized patients.

## Contribution

The study identifies age and care setting as key factors influencing influenza A (H3N2) viral load during the 2022-2023 season in Spain.

## Key findings

- Infants, children, and the elderly have the highest influenza A (H3N2) viral loads.
- Outpatients have higher viral loads than hospitalized patients, likely due to disease phase differences.
- Viral load is marked by lower cycle threshold values in extreme age groups and non-sentinel patients.

## Abstract

•This is a prospective analysis of 1047 patients with A (H3N2) laboratory-confirmed infection.•The analysis of the cycle threshold value by quantitative reverse transcription-polymerase chain reaction was used as a marker of viral load.•Infants, children, and the elderly present the highest viral load.•Outpatients showed higher viral loads than hospitalized patients.•Age and care setting are important for viral load evaluation.

This is a prospective analysis of 1047 patients with A (H3N2) laboratory-confirmed infection.

The analysis of the cycle threshold value by quantitative reverse transcription-polymerase chain reaction was used as a marker of viral load.

Infants, children, and the elderly present the highest viral load.

Outpatients showed higher viral loads than hospitalized patients.

Age and care setting are important for viral load evaluation.

The aim of this study was to analyze the viral load (VL) using cycle threshold (Ct) in patients infected with influenza A (H3N2).

This prospective study was conducted during the 2022-2023 influenza season in sentinel, non-sentinel, and hospitalized patients of Castilla y León (Spain). Respiratory samples were obtained from nasopharyngeal swabs and analyzed by quantitative reverse transcription-polymerase chain reaction specific for influenza A (H3N2) to obtain the Ct value.

A total of 1047 individuals were enrolled (174 [16.6%] sentinel, 200 [19.1%] non-sentinel, 673 [64.3%] hospitalized). The mean Ct value was lower in infants, young children, and in the elderly, with a sharp increase in the last from 65 years until 90 years. In addition, the lower Ct values were observed in non-sentinel patients and then in hospitalized patients, probably because non-sentinel are outpatients in the acute phase of the influenza infection.

A higher VL (lower Ct value) is related to the extreme ages of life: children and the elderly. Furthermore, a higher VL is related with the care setting, being probably higher in outpatients because they are in the acute phase of the disease and slightly lower in hospitalized patients because they are attended during the post-acute phase.