Background Hundreds of plant species release their pollen into the air every year during early spring. During that period, pollen allergic as well as non‐allergic patients frequently present to doctors with severe respiratory tract infections. Our objective was therefore to assess whether pollen may interfere with antiviral immunity. Methods We combined data from real‐life human exposure cohorts, a mouse model and human cell culture to test our hypothesis. Results Pollen significantly diminished interferon‐λ and pro‐inflammatory chemokine responses of airway epithelia to rhinovirus and viral mimics and decreased nuclear translocation of interferon regulatory factors. In mice infected with respiratory syncytial virus, co‐exposure to pollen caused attenuated antiviral gene expression and increased pulmonary viral titers. In non‐allergic human volunteers, nasal symptoms were positively correlated with airborne birch pollen abundance, and nasal birch pollen challenge led to downregulation of type I and ‐III interferons in nasal mucosa. In a large patient cohort, numbers of rhinoviruspositive cases were correlated with airborne birch pollen concentrations. Conclusion The ability of pollen to suppress innate antiviral immunity, independent of allergy, suggests that high‐risk population groups should avoid extensive outdoor activities when pollen and respiratory virus seasons coincide.

A sudden drop in outdoor temperature might activate the annual influenza epidemic in a temperate climate by facilitating aerosol spread in dry air. These conditions also seem to affect the incidence of other respiratory pathogens but not human rhino- or enterovirus, suggesting that routes of infection other than aerosol may be relevant for these agents.

In an outbreak of measles in Gothenburg, Sweden, breakthrough infections (i.e. infections in individuals with a history of vaccination) were common. The objective of this study was to compare measles RNA levels between naïve (i.e. primary) and breakthrough infections. We also propose a fast provisional classification of breakthrough infections. Medical records were reviewed and real-time PCR-positive samples genotyped. Cases were classified as naïve, breakthrough or vaccine infections. We compared clinical symptoms and measles RNA cycle threshold (Ct) values between breakthrough and naïve infections. Sixteen of 28 confirmed cases of measles in this outbreak were breakthrough infections. A fast provisional classification, based on previous history of measles vaccination and detectable levels of measles IgG in acute serum, correctly identified 14 of the 16 breakthrough infections, confirmed by IgG avidity testing. Measles viral load was significantly lower in nasopharyngeal samples from individuals with breakthrough compared with naïve infections (median Ct-values: 32 and 19, respectively, p < 0.0001). No onward transmission from breakthrough infections was identified. Our results indicate that a high risk of onward transmission is limited to naïve infections. We propose a fast provisional classification of breakthrough measles that can guide contact tracing in outbreak settings.

The frequency of viral respiratory pathogens in asymptomatic subjects is poorly defined. The aim of this study was to explore the prevalence of respiratory pathogens in the upper airways of asymptomatic adults, compared with a reference population of symptomatic patients sampled in the same centers during the same period. Nasopharyngeal (NP) swab samples were prospectively collected from adults with and without ongoing symptoms of respiratory tract infection (RTI) during 12 consecutive months, in primary care centers and hospital emergency departments, and analyzed for respiratory pathogens by a PCR panel detecting 16 viruses and four bacteria. Altogether, 444 asymptomatic and 75 symptomatic subjects completed sampling and follow-up (FU) at day 7. In the asymptomatic subjects, the detection rate of viruses was low (4.3%), and the most common virus detected was rhinovirus (3.2%). Streptococcus pneumoniae was found in 5.6% of the asymptomatic subjects and Haemophilus influenzae in 1.4%. The only factor independently associated with low viral detection rate in asymptomatic subjects was age Ն65 years (P ϭ 0.04). An increased detection rate of bacteria was seen in asymptomatic subjects who were currently smoking (P Ͻ 0.01) and who had any chronic condition (P Ͻ 0.01). We conclude that detection of respiratory viruses in asymptomatic adults is uncommon, suggesting that a positive PCR result from a symptomatic patient likely is relevant for ongoing respiratory symptoms. Age influences the likelihood of virus detection among asymptomatic adults, and smoking and comorbidities may increase the prevalence of bacterial pathogens in the upper airways.

A high clinical suspicion combined with radiological imaging aids early diagnosis. Predisposing factors for developing typhlitis were haematologic malignancy and treatment with chemotherapy within 3 weeks of onset. Supportive care, bowel rest including parenteral nutrition, correction of cytopenias and aggressive antimicrobial treatment is essential. Measurements of C-reactive protein in blood may be of benefit when assessing the clinical course.