The retrospective study combined descriptive and analytical components using 24 consecutive months of data from the national respiratory pathogen sentinel network. Participating institutions were tertiary general hospitals in Chengdu, Dazhou, Nanchong, Mianyang, Panzhihua, Zigong and Ya’an, representing the Chengdu Plain, northeastern, northern, southern and Panxi areas of Sichuan. The study population combined 14,970 outpatient or emergency cases of influenza-like illness, defined by fever with cough or sore throat within three days of onset, and 7,466 hospitalized patients with severe acute respiratory infection. Respiratory specimens were tested simultaneously for SARS-CoV-2 and influenza virus using a 17-target microfluidic multiplex real-time RT-PCR assay. SARS-CoV-2 was targeted through the ORF1ab and N genes, while conserved M and NP targets detected influenza A and B, with additional targets distinguishing A(H3), A(H1N1)pdm09, B/Victoria and B/Yamagata.

Of 25,624 records initially exported from the surveillance system, 3,188 came from institutions outside the seven designated hospitals and were excluded; after deduplication and completeness verification, 22,436 valid cases remained. Results were reported as positive when the cycle threshold was below 37, while samples with values from 37 to less than 40 were retested using the original specimen. Retested samples with a cycle threshold of at least 40 were classified as negative, and specimens that remained in the borderline range were treated as suspicious and excluded. Positive, negative and blank controls accompanied each batch, and the network laboratory regularly participated in national and provincial external quality assessments with satisfactory results. Wilson score intervals were used to estimate 95% confidence intervals, while multivariable logistic regression examined associations with age, sex, season, case type and region.

Seasonality was the clearest division between the viruses. SARS-CoV-2 positivity ranged from zero to 17.46% by month and followed a summer-centered pattern, with peaks in July and August 2024 and from May through July 2025. Its highest monthly rate occurred in June 2025, when 17.46% of cases tested positive. Influenza followed a winter–spring pattern, with peaks from December 2024 to January 2025 and from November to December 2025; its highest monthly positivity, 40.30%, occurred in December 2025. Multivariable analysis showed that summer carried much higher odds of SARS-CoV-2 positivity than winter, with an odds ratio of 23.88 and a 95% confidence interval of 17.04 to 33.46. The reverse pattern was evident for influenza, for which summer had substantially lower odds than winter, with an odds ratio of 0.16 and a 95% confidence interval of 0.14 to 0.19.

The age profiles also differed. SARS-CoV-2 positivity was highest among people aged 15 to 34 years, at 7.69%, compared with 3.23% in children aged 0 to 5, 4.45% among those aged 6 to 14, 4.84% at ages 35 to 59 and 3.02% among adults aged 60 or older. In the adjusted model, the 15-to-34 group had 2.32 times the odds of SARS-CoV-2 positivity as the youngest group. Influenza positivity was highest among children aged 6 to 14, reaching 15.17%, compared with 6.15% in those aged 0 to 5, 13.37% at ages 15 to 34, 9.76% at ages 35 to 59 and 6.67% among adults aged 60 or older. The adjusted odds ratio for the 6-to-14 group relative to the 0-to-5 group was 2.52, with a 95% confidence interval of 2.21 to 2.88. Sex differences were modest: female patients had a slightly higher SARS-CoV-2 positivity rate than male patients, while influenza detection did not differ significantly by sex.

Influenza A accounted for 87.5% of the 2,205 influenza-positive cases, including 1,045 A(H3) and 875 A(H1N1)pdm09 infections. Influenza B represented 12.5% of positive cases, almost all of them B/Victoria, with no B/Yamagata detections during the study period. Other respiratory targets were also identified by the multiplex panel: Haemophilus influenzae was detected in 15.59% of cases, Streptococcus pneumoniae in 10.98%, rhinovirus in 8.79% and respiratory syncytial virus in 4.15%. These findings concern nucleic-acid detection across the full study population, not the subset testing positive for either influenza virus, and the analysis did not evaluate interactions involving these other detected pathogens.

Regional measurements showed substantial geographic heterogeneity. SARS-CoV-2 positivity ranged from 3.23% in Chengdu to 6.03% in Zigong, while Panzhihua also had a comparatively high rate of 5.38%. Influenza positivity ranged from 6.12% in Ya’an to 13.07% in Dazhou, with Zigong close behind at 12.98%. In the adjusted model, several regions had higher odds of influenza positivity than Nanchong, including Dazhou, with an odds ratio of 2.53, and Zigong, with an odds ratio of 2.37. Regional contrasts describe differences among the sentinel populations during this observation period; because the study included no province-wide pre-pandemic dual-pathogen surveillance baseline, the data cannot establish how the distributions changed relative to earlier years.

The 11 coinfections occurred during months in which both viruses were detected, but they were far below the level predicted by independent circulation. Six occurred in August 2024, the only month in which SARS-CoV-2 positivity exceeded 5% and influenza positivity exceeded 10%. The other five occurred in December 2025, May 2026 and July 2026. Coinfections were more frequent during the concurrent-circulation month: six of 770 cases, or 0.78%, compared with five among 21,666 cases, or 0.02%, in other months, yielding an odds ratio of 34.02 and a 95% confidence interval of 8.62 to 141.16. Even in August 2024, however, the observed number was below the approximately 13 coinfections expected under independence. Seven of the 11 cases came from Dazhou, where the coinfection rate was 0.21%, significantly higher than in the other sentinel cities. The clustering may reflect genuine local overlap, but the small number of events means it cannot exclude clustering related to specimen submission or testing batches.

The findings do not establish that viral interference caused the negative association. Viral interference, in which one virus induces innate immune responses that inhibit another, provides one possible biological mechanism, and the authors cited experimental evidence that influenza A can enhance SARS-CoV-2 infectivity at the cellular level. Those in vitro observations operate at a different scale from population surveillance: staggered epidemic peaks reduce opportunities for people to encounter both viruses at once, potentially explaining the observed O/E ratio without requiring an individual-level interaction. The analysis was ecological and could not distinguish temporal separation in co-exposure opportunities from direct interference. It also did not include vaccination history, underlying disease or clinical outcomes, so the study could not assess whether coinfection affected illness severity. Its urban, hospital-based sampling further limits generalization because the network poorly represented rural residents, community cases and asymptomatic infections.

Overall, the surveillance data indicate that Sichuan’s two major respiratory viruses followed separate seasonal paths during the two-year window: SARS-CoV-2 was concentrated in summer and among people aged 15 to 34, whereas influenza peaked in winter and spring and was most frequent among children aged 6 to 14. Dual infection remained uncommon, with no population-level evidence of synergistic circulation, despite local and temporal clustering. The authors recommend pathogen- and region-specific surveillance, including simultaneous testing where concurrent circulation is detected, and note that combining pathogen data with information on population immunity, mobility and weather could improve early-warning capacity. These recommendations are prospective implications of the study rather than evidence that the reported interventions have already reduced transmission.

Subject of Research: Seasonal patterns and coinfection of SARS-CoV-2 and influenza virus in Sichuan sentinel surveillance

Article Title: Analysis of multiplex pathogen sentinel surveillance results of SARS-CoV-2 and influenza virus in Sichuan Province, 2024–2026

Article References: Hu, F., Zhou, X., Zhou, L., Li, Z., Dong, S., & Xiao, C. (2026). Analysis of multiplex pathogen sentinel surveillance results of SARS-CoV-2 and influenza virus in Sichuan Province, 2024–2026. Frontiers in Public Health, 14, Article 1961515. https://doi.org/10.3389/fpubh.2026.1961515

Image Credits: AI Generated

DOI: 10.3389/fpubh.2026.1961515

Keywords: SARS-CoV-2, influenza virus, viral coinfection, seasonal surveillance, multiplex RT-PCR, Sichuan Province, acute respiratory infection, sentinel hospitals, population-level association, viral interference, Analysis, multiplex

The research team, led by Dieter J. A. Heylen of the Evolutionary Ecology Group at the University of Antwerp, together with colleagues from the Luxembourg Institute of Health and certified bird ringers in Belgium, examined 284 redwings captured during late-autumn migration in 2022 and 2024. Both years were marked by unusually mild autumn conditions, a detail that proved central to interpreting the results. Of the birds sampled, 72.9 percent were infested with ticks, a rate the authors describe as the highest ever reported for this species anywhere in Europe. The overwhelming majority of the parasites collected were Ixodes ricinus, the castor bean tick whose blood-feeding stages are responsible for transmitting the majority of tick-borne infections to humans and domestic animals across the European continent.

The scale of pathogen carriage among those ticks was equally striking. The team screened 698 ticks removed from the migrating birds and found that 12.9 percent carried Borrelia bacteria. The infections were not confined to a single lineage. Genetic screening identified multiple genospecies of the Borrelia burgdorferi sensu lato complex, including strains associated with birds and others associated with mammals, alongside Borrelia miyamotoi, a relapsing-fever spirochete increasingly recognized as a human pathogen. Perhaps most telling, infected ticks were found on nearly one-third of the birds examined, meaning that a large proportion of the migrating population was effectively functioning as a long-distance courier service for viable, pathogen-laden parasites.

Extrapolating from their field measurements to the size of the European redwing population, the researchers estimate that tens of millions of Borrelia-infected ticks may be transported southward each autumn on the backs of these migrating thrushes. Ticks attached to birds can detach at stopover sites hundreds or thousands of kilometers from where they were acquired, seeding new locations with both the arthropods themselves and the pathogens they carry. Because migratory birds travel enormous distances in short periods, they represent one of the most efficient dispersal mechanisms available to ticks, whose own locomotion over a lifetime rarely exceeds a few meters. The study thus adds a seasonal dimension to what ecologists already understood about birds as tick vectors: the phenomenon does not stop when summer ends.

The timing of the infestations is what gives the work its conceptual force. Migratory birds calibrate the onset and pace of their journeys primarily by photoperiod, the changing length of daylight, which is an astronomical cue immune to year-to-year weather. Tick activity, by contrast, is governed largely by ambient temperature, since ticks are ectotherms whose questing behavior halts when conditions become cold. Historically, late-autumn temperatures across much of western and central Europe fell low enough to push ticks into dormancy well before the main passage of redwings, minimizing the overlap between the two biological calendars. The Belgium study, conducted during two exceptionally mild Novembers, documents what happens when that buffer disappears: host and parasite schedules collide, and the birds depart with an unintended cargo.

This divergence between climatically driven parasite activity and photoperiodically driven migration is a textbook example of a phenological mismatch, and the study’s authors argue that continued climate warming is likely to widen it in the parasites’ favor. Migration timing is comparatively stable, anchored to day length, while the tick activity season expands at both ends of the year as winters warm. Each increment of autumn warming therefore extends the period during which questing ticks can board migrating hosts. The consequence, according to the researchers, is a strengthening seasonal window for long-distance parasite dispersal, one that epidemiological models of Lyme disease risk have largely failed to incorporate because they treat late autumn as an epidemiological dead zone.

The age structure of the infestations added a further layer of biological insight. Birds in their first calendar year, that is, individuals hatched the same summer and undertaking their first migration, carried significantly higher tick loads than older birds. The authors suggest several non-exclusive explanations, including the inexperience of young birds in grooming and preening parasites effectively, behavioral differences in habitat use that bring first-year birds into closer contact with questing ticks, and possible physiological differences in skin characteristics that affect how readily ticks attach and feed. Whatever the mechanism, the pattern means that a disproportionate share of the dispersing parasite burden travels on the youngest cohort, the very individuals that make up the bulk of many migratory populations and that colonize wintering areas before returning north in spring.

The pathogens detected on these birds are of more than academic concern. Borrelia burgdorferi sensu lato encompasses the genospecies responsible for the great majority of Lyme borreliosis cases in Europe, an infection that can produce erythema migrans rashes, neurological complications, arthritis and cardiac involvement when untreated. Borrelia miyamotoi, though historically overshadowed by its Lyme-causing relatives, has been implicated in human cases of a relapsing-fever-like illness and, in immunocompromised patients, severe meningoencephalitis. The identification of both avian-associated and mammalian-associated genospecies on migrating redwings underscores the role of birds in maintaining and redistributing genetically diverse pathogen reservoirs, with consequences for the local disease ecology of every stopover and wintering ground along the flyway.

It is important to note that the study does not claim that migrating birds directly infect humans. Ticks feeding on birds do not transmit Borrelia to their hosts in ways that pose immediate public-health danger at the moment of capture. The risk pathway is indirect: infected ticks detach in new environments, and if those environments support the tick’s life cycle and suitable reservoir hosts, the parasites and their pathogens can establish locally. A southbound redwing that drops an infected larva or nymph in a French hedgerow, an Iberian woodland or a Mediterranean scrubland effectively introduces Borrelia genetic material into a new ecological arena, where subsequent feeding cycles on local wildlife can amplify and maintain the pathogen. The study’s contribution is to show that this introduction channel remains open deep into the season previously considered safe.

The work also carries a methodological message for disease surveillance. Monitoring programs for tick-borne pathogens in Europe typically concentrate their efforts on spring and early summer, when human exposure risk peaks and tick activity is maximal. The Belgian findings argue for extending surveillance into the late-autumn migration window, particularly during warm years, to capture a dispersal event of continental significance. With both sampling years of the study falling among the mildest autumns on record, and with climate projections pointing toward more frequent such conditions, the overlap between southbound migration and active, infected ticks may shift from an anomaly to a routine feature of the European disease landscape. What the redwings reveal, in effect, is that the geography and seasonality of Lyme disease risk are being quietly rewritten at altitude, one migrating thrush and one hitchhiking tick at a time.

Subject of Research: Late-autumn dispersal of Borrelia-infected ticks by southward-migrating redwings and its implications for Lyme disease epidemiology under climate warming

Article Title: Lyme beyond the season: late-autumn dissemination of infected ticks by southward migrating redwings (Turdus iliacus)

Article References: Heylen, D. J. A., Ledegen, I., Grimon, M., Reteng, P., Hübschen, J. M., & Krawczyk, A. I. (2026). Lyme beyond the season: late-autumn dissemination of infected ticks by southward migrating redwings (Turdus iliacus). Parasites & Vectors. https://doi.org/10.1186/s13071-026-07673-x

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07673-x

Keywords: Lyme disease, redwing, Turdus iliacus, Ixodes ricinus, Borrelia, bird migration, tick-borne pathogens, phenological mismatch, climate change, parasite dispersal, Borrelia miyamotoi, seasonal surveillance