The 2023-24 influenza vaccine was most protective in the 2 weeks to 2 months following vaccination, and more effective in preventing influenza among adults aged 18 to 49 years compared to older adults, according to a recent study.

The study, published in The Journal of Infectious Diseases, was based on cases seen in a sentinel surveillance network of 24 medical centers in 20 states from September 1, 2023, to May 31, 2024.

In total, there were 7,690 patients hospitalized with acute respiratory illness (ARI) during the study period, including 1,170 flu cases (33% vaccinated) and 6,520 controls. 

Protection waned by 4 months 

Overall, vaccine effectiveness (VE) was 40% (95% confidence interval [CI], 31% to 48%). The vaccine was most effective in adults ages 18 to 49 years (53%), compared with those 50 to 64 years (47%) and those 65 and older (31%).

VE waned significantly; at 14 to 60 days post-vaccination, VE was 54%. VE dropped to 18% after 120 or more days. 

Notably VE was statistically significant against influenza B (67%) and A(H1N1) (36%) abut not against A(H3N2). All three influenza strains circulated during the 2023-24 season.

Analyses by virus subtype and time since vaccination showed reduced protection among patients who had influenza A(H3N2).

"Analyses by virus subtype and time since vaccination showed reduced protection among patients who had influenza A(H3N2) and among patients vaccinated >120 days prior to enrollment, the latter of which was likely attributable to waning immune response rather than antigenic drift of the circulating viruses," the authors wrote. 

A study of hospitalized patients with peripheral intravenous catheters (PIVCs) found a significantly increased risk of bloodstream infections (BSIs) after 3 days of dwell time, Swiss researchers reported today in JAMA Internal Medicine.

The cohort study, conducted at Geneva University Hospitals in Switzerland, involved 371,061 patients with at least one PIVC inserted on the upper extremities from January 2016 through February 2000. Although BSIs linked to PIVCs are rare, recent research has shown that the use of PIVCs is widespread in hospitals, and the authors of the study note that BSIs with PIVCs are associated with increased mortality and substantial infection-related costs. For this study, they wanted to investigate whether dwell time (the length of time the PIVC is inserted in a patient) is linked to development of BSIs.

Significantly higher risk after 3 days' dwell time

A total of 140,178 patients (38%) had a PIVC dwell time of 1 to 2 days, 119, 252 (32%) had a dwell time of 3 to 4 days, and 111, 631 (30%) had a dwell time of more than 4 days. The instantaneous risk of BSIs with PIVCs was low in the first 2 days of dwell time and increased rapidly thereafter. Multivariable risk models showed the risk of BSIs was increased 13-fold after 3 days of catheter maintenance compared with 3 or fewer days (adjusted odds ratio [AOR], 13.55; 95% confidence interval [CI], 5.44 to 34.00).

While the highest risk was observed after 3 days, increased risk was also observed after 4 days (AOR, 8.53; 95% CI, 4.47 to 16.28), 5 days (AOR, 5.38; 95% CI, 3.23 to 8.96), and 6 days (AOR, 7.63; 95% CI, 4.57 to 12.74). Most BSIs (80%) were identified after 4 days.

The authors say the results have important clinical implications

"Based on these results, PIVC maintenance therapy should be challenged at day 3, and PIVC replacement should be at least considered; PIVC replacement should be performed at day 4," they wrote.