Hospitalisations and deaths associated with influenza are largely caused by associated complications including pneumonia and acute exacerbation of underlying conditions like cardiopulmonary diseases and acute renal failure. However, pneumonia is the most common and serious complication associated with influenza. Following influenza infection, pneumococcal pneumonia is particularly common.¹ Although regulation of secondary pneumonia infection following the incidence of influenza remains unclear, it has been observed that prior influenza infection affects pneumococcal adherence and invasion. This highlights the need for preventive measures against both influenza and secondary pneumococcal infections. Evidence indicate that rather than limiting secondary pneumococcal infection, limiting influenza infection could be more effective in preventing pneumococcal pneumonia in these patients.²

The association between the bacterium S. pneumoniae and the influenza virus has been proposed as a polymicrobial system, where the pathogenicity and transmission of one pathogen is affected by interactions with the other pathogen.³ A study evaluated the role of influenza in the epidemiology of pneumonia using a mechanistic transmission model and reported that influenza infection leads to a 100-fold increase in the risk of pneumonia.⁴ Similar findings were reported by another study, which revealed that there exists a strong but short-lived interaction between the 2 pathogens and that influenza infection leads to around a 100-fold increase in the susceptibility to pneumococcal pneumonia.³

In the Indian setting, data on the contribution of influenza leading to CAP in adult population are scarce. A study of adults from North India hospitalised with CAP assessed the microbial aetiology of CAP in these patients. This study involved 225 adults with the median age being 59 years. S. pneumoniae (30.5%) and influenza viruses (15.4%) were among the most common aetiological agents for these adults with CAP. In those cases where the aetiology of pneumonia was polymicrobial, influenza was found to be a co-pathogen in 33% of such patients.⁵

Since pneumonia and exacerbations of the underlying chronic illness are the major causes of influenza-related hospitalisations, use of both pneumococcal and influenza vaccines is strongly recommended in the adult population with comorbidities.⁶ A Phase 3, randomised, parallel-group, multicentre trial conducted across 34 sites in the United States evaluated the safety, immunogenicity and tolerability of PCV13 co-administered with TIV in pneumococcal vaccine-naïve adults. This study enrolled adults aged 50 to 59 years. It revealed that the co-administration of PCV13 with TIV in this group of adults was well tolerated and immunogenic. However, co-administration of the 2 vaccines was associated with a relatively lower antibody response for PCV13 as compared to the administration of PCV13 alone, the clinical significance of which was unknown. This study concluded that owing to the positive immunogenic attributes of PCV13, co-administration of the 2 vaccines should be considered, particularly in the clinical setting.⁷ Another randomised controlled trial assessed the effects of co-administration of PCV13 with TIV in PPSV23-naïve adults aged ≥65 years. This study reported that the co-administration of both the vaccines demonstrated acceptable safety and immunogenicity as compared to either of the vaccines given alone.⁸

Another randomised study evaluated the response of PCV13 co-administered with QIV in adults aged ≥50 years and previously immunised with PPSV23. The immune response for co-administered vaccines was non-inferior as compared to either of the vaccines given alone, albeit lower for co-administered PCV13. This study concluded that the concomitant use of PCV13 and QIV can be considered in this adult population, when they are pre-immunised with PPSV23.⁹ A Phase 4 study reported that the concomitant use of PCV13 and QIV yielded a similar immune response as compared to QIV alone and was well tolerated. Although the immune response was non-inferior to that of PCV13 alone, lower PCV13 responses were observed in the co-administration group of adults.¹⁰ All these studies confirmed the safety and immunogenicity of co-administration of PCV13 and either of the influenza vaccines, TIV or QIV, in the older adult population.

Several studies have described the impact of PCVs in preventing influenza-associated morbidity and mortality in children. An analysis of adults aged ≥65 years estimated the efficacy of PCV13 in the prevention of hospitalisations due to CAP-associated influenza. While in the PCV group, 23 cases of CAP-associated with influenza were observed, the number was 35 in the placebo group, with a 34.4% vaccine efficacy. However, since this study was a secondary analysis for which the original trial was not powered, the PCV efficacy was not statistically significant.^11,12

Vaccinations against both influenza and pneumococcal infections are recommended for preventing CAP in adults. Several national and global guidelines recommend the concomitant use of pneumococcal and influenza vaccines in the adult population.¹³ The CDC recommends the co-administration of a pneumococcal vaccine (either PCV13 or PPSV23) with an influenza vaccine during the same visit at different sites.¹⁴ CDC recommend the co-administration of both the vaccines, but administering at different sites.¹³