Improving detection of secondary immunodeficiency and vaccination after invasive airway infections: a prospective cohort study with retrospective comparison

BACKGROUND: Infections caused by encapsulated pathogens such as Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis contribute to significant morbidity and mortality, with immunodeficiency being an important risk factor for recurrent and invasive disease. A previous retrospective Australian study identified a 6% rate of newly diagnosed immunodeficiency in such patients and highlighted inadequate screening. METHODS: A prospective study at a large multi-centre health service in South-East Melbourne, Australia was conducted between June 2021 and September 2023. Adults (≥16 years) with the aforementioned organisms, isolated from sterile sites, were recruited. Participants underwent basic immunodeficiency testing and vaccinations. A retrospective cohort, comprising eligible but unenrolled patients over the same period, was retrospectively analysed for comparison. RESULTS: Patients within the prospective cohort (n = 46) were compared with the retrospective cohort (n = 69), with the predominant organisms being S. pneumoniae (83%, 94/115), H. influenzae (13%, 15/115) and N. meningitidis (4%, 5/115). Vaccine-preventable strains accounted for 75% (55/73) of typed cases. A new diagnosis of immunodeficiency was identified more frequently in the prospective group (7/46, 15.2% vs 4/69, 5.8%, P = 0.1), with a numerically greater, though not statistically significant, increase in immunodeficiency detection rate seen in the prospective cohort (OR 2.9, 95% CI 0.7-14.3, P = 0.1). Post-illness vaccinations were more frequently recommended in the prospective group (OR 2.6, 95% CI 1.0-64, P = 0.03). CONCLUSION: Actively identifying patients with invasive airway pathogens and ensuring linkage with an infectious diseases service improves screening and diagnosis of immunodeficiencies and optimises vaccination opportunities.