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Influenza A and B co-infection: a case–control study and review of the literature


Influenza virus infection remains a major cause of morbidity and mortality during winter seasons. Bacterial and virus co-infection is a commonly described situation in these patients. However, data on co-infection by influenza A and B viruses are lacking. In this study, we present the cases of co-infection by influenza A and B viruses during the winter season of 2014–2015 in our institution. We analyzed 2759 samples from 2111 patients and found that 625 samples corresponding to 609 patients were positive for influenza A or B virus. A total of 371 patients had influenza A, 228 had influenza B, and 10 (1.6 %) had influenza A and B virus detection in the same sample. The median age of co-infected patients was 78.6 years, and only one of the co-infected patients died because of the infection. Comparison with a control group of mono-infected patients revealed that co-infection was significantly associated with nosocomial acquisition [odds ratio (OR) = 4.5, 95 % confidence interval (CI) = 1.05–19.25, p = 0.042]. However, co-infection was not associated with worse outcome, previous underlying condition, or vaccination status. Multivariate analysis revealed that co-infection was not an independent risk factor for death and that no single risk factor could predict co-infection.


Influenza and other respiratory pathogens are a major cause of morbidity and mortality worldwide during winter. A significant number of patients with influenza are co-infected with another pathogen [1], mainly bacteria such as Streptococcus pneumoniae [2].

In recent years, with the introduction of molecular diagnostic techniques, co-infection of influenza and other viral pathogens is becoming more frequently diagnosed. The most common association is with respiratory syncytial virus, especially in children [3]. Influenza A and B co-infection has been only occasionally reported [46], and the clinical significance of mixed influenza infections remains unclear.

In this study, we report ten cases of co-infection by influenza A and B during the 2014–2015 influenza season in our institution and compare them with cases infected by only one influenza virus. We also performed a systematic review of the literature to identify previous cases of co-infection by influenza A and B viruses.

Materials and methods

Settings and study design

We conducted the study in a large general teaching hospital with 1550 beds and a catchment population of approximately 715,000 inhabitants in Madrid, Spain.

We performed a retrospective study of all respiratory samples from patients with suspicion of influenza delivered to our laboratory from December 2014 to March 2015.

Diagnostic procedures

We selected episodes of influenza A and B co-infection, defined as isolation of both viruses in the same sample from a symptomatic patient.

Respiratory samples with suspicion of infection by influenza virus were processed according to our diagnostic algorithm [7], which is based on a rapid antigen detection test (RADT) (Xpect™ Flu A&B, Remel Europe Ltd.), followed by a reverse transcriptase polymerase chain reaction (RT-PCR) assay (RT-PCR Flu A/B Typing Real-time Detection, Anyplex) and/or a shell vial assay in two cell lines (MRC-5 and A-549).

Inclusion and exclusion criteria

We included all cases of influenza confirmed by cell culture and/or RT-PCR. All cases with an initial positive RADT result that could not be confirmed were excluded. Co-infection was defined as the detection of influenza A and influenza B viruses in the same sample simultaneously.

All cases of co-infection were recorded and clinical data were retrieved from the electronic medical records. Clinical data included age, gender, underlying disease and other comorbidities, cause of admission, prescription of antiviral treatment, outcome, length of hospital stay, intensive care unit (ICU) admission, and place of acquisition.

Infection was considered nosocomial if symptoms developed at least 48 h after admission and community-acquired if symptoms started before or within 48 h of admission.

We then selected four mono-infected patients per co-infected patient as controls and recorded the same medical data. We compared mono-infected patients with co-infected patients in order to determine whether co-infection was associated with a poor prognosis.

The control group included patients with a positive sample for influenza A or B by RT-PCR and whose sample was submitted to our laboratory 2 days before or 2 days after the arrival of a sample from a co-infected patient.

Literature review

We performed a literature review for articles in English, French, and Spanish using the medical subject headings “influenza A and influenza B co-infections” and results filtered by “human species”. We included all cases of influenza A and B co-infection that had a positive result by RT-PCR and/or cell culture.

Statistical analysis

Comparisons between co-infected and mono-infected patients were made using the t test or Mann–Whitney test for continuous variables, depending on the normality of the distribution. Differences between categorical variables were studied using a two-tailed Fisher’s exact test. A p-value less than or equal to 0.05 was considered significant, and results were expressed as odds ratios (ORs). The 95 % confidence intervals (95 % CIs) were calculated for each risk factor using the Wald approximation method. A logistic regression model was used to analyze the associations between the variables of interest. The results were also expressed as the OR and 95 % CI. Logistic regression was used to establish which variables could predict co-infection. The statistical analysis was performed using Stata/IC 13.1 (StataCorp, College Station, TX, USA)



A total of 2759 samples from 2111 patients were delivered to our laboratory during the study period. Our algorithm showed that 625 (22.7 %) samples from 609 patients were positive for influenza A virus (60.2 %) or influenza B virus (39.8 %).

Co-infection was detected only in ten samples (1.6 %), corresponding each sample to a unique patient. We excluded one patient who had a positive RADT result for both influenza A and B without confirmation by culture or RT-PCR.

Description of cases

The demographic characteristics of the ten patients are summarized in Table 1.

Table 1 Demographic characteristics of the ten co-infected patients

The median age of co-infected patients was 78.6 years [interquartile range (IQR): 42.3–83.8]. Five patients were male. The most frequent underlying diseases were ischemic heart disease (3 of 10), chronic obstructive pulmonary disease (2 of 10), and chronic kidney disease (2 of 10). One patient had sarcoma, another had gastrointestinal disease, and, finally, there was one patient without any comorbidities or underlying diseases.

The results of the comparison between the groups are presented in Table 2. There were no significant differences in age, sex, or comorbidities. Vaccination status was only available in seven co-infected and 20 mono-infected patients. Only one co-infected patient was vaccinated. There were no statistical differences between the groups (p = 0.09).

Table 2 Comparison between co-infected and mono-infected patients

Acquisition was nosocomial in six co-infected patients and healthcare-related (nursing home) in one patient. Nosocomial acquisition was more common in co-infected patients than in mono-infected patients (60 % vs. 25 %, OR = 4.5, 95 % CI = 1.05–19.25, p = 0.042).

Only four out of ten co-infected patients received antiviral drugs, and there were no statistical differences for this parameter between co-infected and mono-infected patients (p = 0.150). The median length of stay (LOS) was 11 days (IQR: 3–15) in co-infected patients and 7 days (IQR: 4–13) in mono-infected patients, although the difference was not statistically significant (p = 0.836).


All the cases of co-infection were diagnosed using RT-PCR. The RADT results were negative for both viruses in all cases except in patient number 6, whose result was positive only for influenza B.


Only one co-infected patient had to be admitted to the ICU because of infection. Seven out of ten patients completely recovered from the infection. Two patients died of causes not directly associated with the infection, and only one death was related to the infection. We could not find differences in outcome or in the ICU admission rate (10 % vs. 17.5 %, OR = 0.52, 95 % CI = 0.06–4.83) between co-infected or mono-infected patients. Mortality was 10 % in both groups (OR = 1, 95 % CI = 0.10–10.07).

Risk factors associated with mortality

The results of the univariate analysis of risk factors associated with mortality are summarized in Table 3. After adjusting for the effects of potentially confounding variables in a logistic regression model, we found that ICU admission was the only predictor of mortality (OR = 9.98, 95 % CI = 1.16–86.13). Nevertheless, co-infection was not a risk factor in our study population.

Table 3 Risk factors associated with mortality in mono-infected and co-infected patients in the univariate analysis

We performed a multivariate analysis to determine which risk factors could be associated with co-infection and found that none of the study variables was an independent risk factor for co-infection. Nevertheless, it is noteworthy that nosocomial acquisition was the only variable that almost reached statistical significance (p = 0.054, OR = 4.89, 95 % CI = 0.98–24.58).

Literature review

Our literature review disclosed 45 cases of co-infection [46, 813] (Table 4).

Table 4 Cases of influenza A and B co-infection in the literature

Data on vaccination status were available for only five patients, of whom only one had previously been vaccinated.

The PCR results for both influenza viruses were positive in all cases.

Data on the underlying condition were available for only three patients, all of whom were immunosuppressed (one leukemia, one solid organ recipient, one HIV-infected). Outcome was poor in 8 out of 45 patients (ICU admission or death, data not specified).


Our study shows that co-infection with influenza A and B viruses is unusual and mainly nosocomial. However, the prognosis of co-infection is not worse than that for mono-infection.

Viral influenza is a seasonal infection that is associated with considerable morbidity and mortality [14], and usually involves other respiratory pathogens [1], mainly bacteria [2]. Molecular diagnostic tests have greatly facilitated the detection of co-infection by influenza and other viral pathogens [3]. However, co-infection by influenza A and B, defined as the detection of both viruses in the same sample simultaneously (i.e., not sequential infections), is very rare. This lack of data may be explained with two simple facts. First, circulation of both viruses simultaneously during the same season is unusual. Two years ago, most cases of influenza infection were caused by influenza A; however, during the last season, both viruses were circulating simultaneously [15]. Second, simultaneous infection with two influenza viruses is highly unusual. We found only two studies reporting data on rates of co-infection. Falchi et al. [4] reported a rate of 3.2 % and Goka et al. [11] a rate of 7.3 %.

In our study, 1.6 % of patients were co-infected, indicating that co-infection is a rare event. This value was lower than that reported by Goka et al. [11]. Our study cannot be compared with that of Goka et al. [11], since the study populations and periods were different. The study of Goka et al. [11] included almost 26,000 samples (mainly pediatric) over a longer period (4 years), which also covered the outbreak of pandemic influenza A (H1N1).

Few studies have examined the clinical impact and risk factors of influenza A and B co-infection in adults. Goka et al. [11], who were the only authors with a sufficient number of patients to establish the clinical impact of co-infection, concluded that influenza B co-infection in patients with seasonal influenza A virus infection significantly increased the risk of ICU admission or death. However, the main limitations of the study were that the authors did not measure the impact of other covariates, such as the underlying diseases, and the fact that most co-infections (approximately three-quarters) occurred in children aged <5 years.

We found that co-infection does not indicate a poor prognosis and is not associated with higher mortality, greater risk of ICU admission, or longer hospital stay. Moreover, one of the strengths of our study is that we analyzed the impact of other covariates and found that nosocomial acquisition was more frequent in co-infected than in mono-infected patients. Nevertheless, the low number of cases of co-infection and the results of the logistic regression analysis prevent us from predicting that a patient with a nosocomial infection will be co-infected by both viruses.

Our study is limited by its retrospective design and the fact that it was conducted over one season in a single hospital. The changes in the epidemiology of circulating influenza viruses prevent our results from being extrapolated to future seasons.

In summary, influenza A and B co-infection seems to be a rare entity that is usually nosocomially acquired and not associated with a poorer prognosis than mono-infection. To our knowledge, this is the first case–control study that analyzes epidemiology, risk factors, complications, and outcome in patients co-infected with influenza A and B. Further comparative studies with a higher number of cases are needed.


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We thank Thomas O’Boyle for his help with the preparation of the manuscript.

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Corresponding authors

Correspondence to F. Pérez-García or E. Bouza.

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B. Rodríguez-Sánchez is supported by Miguel Servet contract MS14/00220, which is part of the Plan Estatal I+D+I 2013-2016, co-financed by ERDF (FEDER) Funds from the European Commission, “A way of making Europe”.

Conflict of interest

The authors declare that they have no conflicts of interest.

Ethical approval

This study was evaluated and approved by the local ethics committee (Comité Ético de Investigación Clínica Hospital General Universitario Gregorio Marañón Área 1).

Informed consent

Since the present work is a retrospective study, informed consent was not required.

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Pérez-García, F., Vásquez, V., de Egea, V. et al. Influenza A and B co-infection: a case–control study and review of the literature. Eur J Clin Microbiol Infect Dis 35, 941–946 (2016).

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  • Chronic Obstructive Pulmonary Disease
  • Influenza
  • Influenza Virus
  • Respiratory Syncytial Virus
  • Reverse Transcriptase Polymerase Chain Reaction