# The Increased Risk for Postinfluenza Pneumonia Among Cystic Fibrosis Carriers—A Population-Based Study

## TL;DR

This study shows that people who carry the cystic fibrosis gene are at higher risk of developing pneumonia after having the flu compared to those who do not carry the gene.

## Contribution

The study identifies a previously underappreciated increased risk of postinfluenza pneumonia among cystic fibrosis carriers using a large population-based analysis.

## Key findings

- CF carriers had a 34-38% higher odds of developing pneumonia after influenza compared to noncarriers.
- The incidence of pneumonia was 55% greater among CF carriers compared to controls, despite no increased influenza incidence.
- The findings suggest a substantial public health impact due to the high number of CF carriers and annual influenza cases.

## Abstract

Influenza is strongly associated with an increased risk for subsequent bacterial pneumonia. Moreover, cystic fibrosis (CF) carriers are at increased risk for some pulmonary infections. The purpose of this study was to determine whether CF carriers are at greater risk for postinfluenza pneumonia than noncarriers.

Using MarketScan insurance claims data (2001–2023), we identified a study cohort of 38 047 CF carriers and a cohort of 380 470 matched controls. We conducted 2 analyses using these cohorts. First, we assessed individual-level risk for experiencing pneumonia following an influenza infection. Second, because many cases of influenza often do not result in medical visits, we conducted a cohort-level analysis comparing the weekly incidence of pneumonia between CF carriers and noncarriers while accounting for weekly Centers for Disease Control and Prevention–reported influenzalike illnesses across multiple influenza seasons.

At an individual level, we found that the odds of developing pneumonia following a diagnosis of influenza were approximately 34% greater among CF carriers compared with noncarriers. Second, we found that while the incidence of influenza is not elevated among CF carriers, the incidence rate of pneumonia was about 55% greater among CF carriers compared with our control population.

Because 2%–11% of the population acquires influenza each year, and because there are >10–15 million CF carriers in the United States alone, a substantial number of cases of secondary pneumonia may be attributable to the CF carrier state.

Influenza is strongly associated with an increased risk for subsequent bacterial pneumonia. We found that the odds of developing pneumonia following a diagnosis of influenza were approximately 38% greater among CF carriers compared with noncarriers.