COVID-19 Public Health: Seasonal Influenza

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Open article published on MedShr: 21st October 2020
Last updated: 21st October 2020

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Burden of Influenza:

Seasonal influenza, also known as ‘flu’, remains a significant cause of morbidity and mortality globally, accounting for between 290,000 to 650,000 respiratory deaths annually. [1,2,3

  • Pathogen: there are 4 subtypes, of which influenza A and B viruses are most important in humans. Only influenza A viruses have caused previous pandemics; the last influenza pandemic was in 2009 caused by influenza subtype A(H1N1)pdm09 [3]

  • Epidemiology: all age groups are affected, but high risk groups for severe infection include the elderly, young children (especially <5 years), individuals with chronic conditions or immunosuppressive conditions, and pregnant women. Healthcare workers are at high risk of contracting and transmitting influenza.[3]

  • Management and Prevention: for patients with severe infection, oseltamivir (a neuraminidase inhibitor) is recommended, ideally as soon after symptom onset as possible. Vaccination is recommended globally for specific groups to reduce incidence and disease severity, and is administered annually to reflect changes in the prevalence of different influenza virus strains. [3,4]

The WHO provides an informative and concise summary of annual epidemics associated with seasonal influenza.[3]

Recommendations for influenza vaccination vary between countries; for example in the US the CDC recommend, with few exceptions, that everyone over the age of 6 months be vaccinated,[5] whereas in the UK the vaccine is available for identified at-risk groups and all healthcare providers.[6] As cases of SARS-CoV-2 (COVID-19) virus continue to rise globally, public health organisations in the Northern Hemisphere are launching annual flu vaccination campaigns to try and limit the burden of influenza this winter.[2,5,6]

This article aims to provide an overview on the clinical interaction of influenza and COVID-19, discuss the effects of non-pharmacological interventions for COVID-19 on the transmission of influenza, and finally consider the challenges for delivering annual influenza vaccinations during the pandemic.

Figure 1 | Flu-related population mortality rates for those in risk groups in the UK, from the Public Health England Health Matters edition focussing on the flu immunisation programme and COVID-19 (licensed under the Open Government Licence v3.0, see references at end of article)

Clinically Differentiating COVID-19 and Influenza

Influenza and SARS-CoV-2 share several common clinical features including fever, cough, dyspnoea, myalgia and coryzal symptoms, and also have similar biochemical findings of leukopenia, lymphopenia, and elevated CRP.[7,8]

It is therefore challenging to differentiate between the two conditions clinically, and the development of rapid diagnostic testing, preferably point-of-care testing, which can combine assays for SARS-CoV-2, influenza and possibly other respiratory viruses (e.g. RSV) is therefore likely to prove essential.[9,10,11]

Differentiating between influenza and SARS-CoV-2 has several clinical implications:

  • Treatment options differ between the two conditions, for example oseltamivir can be used for severe influenza but steroid use can risk bacterial or fungal superinfection [3]; conversely current evidence supports using dexamethasone in severe SARS-CoV-2 infection [12]. 

  • There are recognised differences in the typical clinical courses, which is important for informing patients of expected symptoms, worsening advice, and monitoring for potential complications. This is also important for providing correct guidance on necessary containment measures (e.g. isolation, return to activities etc.) [9,13]

  • Differences in how the SARS-CoV-2 and influenza viruses affect paediatric patients compared to adults are likely to also be important for predicting both disease severity and also risk of transmission. For example, previous research suggests children have a leading role in propagating influenza outbreaks, whereas the role of children in the spread of SARS-CoV-2 is still not clear.[9,14]


Studies have reported frequent but variable rates of co-infection with SARS-CoV-2 and other respiratory viruses.[15,16,17,18,19] However, the rates of co-infection with influenza specifically is highly variable, and it is not yet clear what effect co-infection with influenza and COVID-19 has on disease severity.[7,8,11,20,21] Rates of co-infection may vary according to seasonal and geographical differences between studies, and therefore further research is needed to fully answer this question. It will be essential to also consider how infection with one virus may influence susceptibility to infection with other respiratory viruses.[11,22]

Effects of Public Health Measures from COVID-19 on Influenza Transmission

Non-pharmacological interventions introduced to try and control the spread of SARS-CoV-2 via respiratory droplets, such as social distancing, facial coverings, promoting regular handwashing, remote working and restricting large gatherings, may be expected to also reduce the transmission of influenza which is also spread by respiratory droplets.[9,10] Several studies have concluded that non-pharmacological interventions initiated in response to COVID-19 have also reduced the transmission of influenza. [23,24]  

Interestingly, analysis of the influenza season in the Southern Hemisphere in 2020 suggests very low circulation in comparison to previous seasons, and similar patterns have been observed in the Northern Hemisphere for inter-seasonal (i.e. summer) monitoring of influenza rates.[10,25] However, authors emphasise that although this could be attributed to decreased transmission from non-pharmacological interventions for COVID-19, some caution must be maintained as testing for non-SARS-CoV-2 respiratory viruses may have been lower earlier in the pandemic,[9] health-seeking behaviours could have changed,[10,25] and influenza circulation can be influenced by other factors such as temperature and virulence.[24]

Influenza Vaccination in the Context of COVID-19

Despite reduced levels of influenza in the Southern Hemisphere, the WHO Strategic Advisory Group of Experts (SAGE) have emphasised the importance of ensuring adequate vaccination coverage for groups at risk of influenza this season in the Northern Hemisphere.[26

It is hoped that sufficient vaccination against influenza will: [2,26]

  • Protect groups, especially the elderly, who are particularly vulnerable to both COVID-19 and influenza, and therefore alleviate additional pressures on healthcare systems 

  • Reduce admissions for influenza treatment and therefore reduce the risk of acquiring COVID-19 in healthcare settings

  • Reduce infection rates amongst healthcare professionals (HCPs), thus both reducing transmission to patients and ensuring the healthcare workforce is not reduced

Interestingly, two preprint studies (which have not yet been peer-reviewed) suggest that influenza vaccination may also decrease mortality from COVID-19,[27,28] but further investigation is required to examine this association.

Influenza vaccination coverage varies significantly both between countries and within different populations.[29] Achieving sufficient influenza vaccination coverage during the current pandemic therefore poses several challenges, and approaches to address these have been proposed in the literature, including:

  • Advanced purchase commitments from governments for influenza vaccines from manufacturers to ensure an adequate supply [30]

  • Targeted educational and public health campaigns dispelling myths about influenza vaccination and explaining the benefits, including the role of protecting not only the individual vaccinated but also family members, colleagues and vulnerable groups within populations.[13,30] Learning from this approach could also help guide campaigns for a COVID-19 vaccine. Importantly, campaigns need to recognise and address challenges of lower vaccination coverage in certain communities, particularly as research has shown that Black, Asian and Minority Ethnic (BAME) groups have been disproportionately affected by COVID-19 [6,31] but may have lower vaccination uptake rates for other conditions.[29,32]

  • Ensuring high uptake of influenza vaccine by healthcare providers is essential; some authors have suggested that mandatory vaccination policies for HCPs may be required [6][13]. The NHS provide a framework based on five key components which aims to promote vaccination uptake by health and social care workers.[33]

  • Promoting safe access for vaccination, especially for vulnerable groups - see following section [30]

  • Encouraging vaccination in institutions such as colleges and universities, or in workplaces;[30] of note, this must be considered in the context of the recommendations by SAGE which aim to ensure adequate supply of influenza vaccines for high-risk populations [26]

  • Increasing childhood vaccination against influenza, both to reduce morbidity and mortality in this at-risk group, but also to reduce the spread of influenza to more vulnerable populations, particularly the elderly. However research has shown parental decisions to vaccinate against influenza may be influenced by the COVID-19 pandemic in many ways.[34,35]

Figure | Safe ways of delivering the flu immunisation programme, from the Public Health England Health Matters edition focussing on the flu immunisation programme and COVID-19 (licensed under the Open Government Licence v3.0, see references at end of article)

Methods for Safely Delivering Vaccination During the Pandemic

Delivering a large-scale vaccination programme during the COVID-19 pandemic raises several challenges:

  • Logistical challenges: such as maintaining social distancing, ensuring adequate supplies of both vaccines and equipment to administer them safely (e.g. needles, syringes, PPE etc.), and planning for staffing requirements to deliver the programme.[2,36] The UK has passed legislation allowing more healthcare providers, including paramedics and midwives, to be trained to administer vaccinations.[37]

  • Clearly communicating: it will be important to ensure patients are still invited for vaccination even when lockdown measures may be in place, and that clear advice relating to PPE requirements and what to expect at appointments are provided in advance.[2,6]

  • Addressing patient concerns: concerns regarding risk of contracting COVID-19 at vaccination appointments, particularly for people in high-risk groups, are understandable and therefore it is important that in addition to clear communication, reassurance is also sought by considering novel methods of delivering vaccinations such as drive-through and mobile or open-air clinics in larger venues.[2,6,36] These novel approaches, however, also bring new logistical challenges.[36]

Reviewing approaches to mass vaccination for influenza during the pandemic will also provide important learning opportunities for planning for mass vaccination if and when effective vaccines against SARS-CoV-2 are available.

Several useful resources providing information and guidance on this topic include:

WHO Strategic Advisory Group of Experts (SAGE) on Immunization for influenza vaccination during the COVID-19 pandemic

WHO Regional Office for Europe

Public Health England

Royal College of General Practitioners (UK)

Royal College of Nursing (UK)

Useful web resources:

WHO: Influenza Updates: Influenza Updates and FluNet 

WHO Eastern Mediterranean (EMRO): Seasonal Influenza

WHO Africa (AFRO): Influenza

Public Health England: for Patients and for Healthcare Professionals

Vaccine Knowledge Project (University of Oxford)

WHO (Europe): Influenza Site 

NHS: Flu vaccine 

CDC: Who Should and Who Should NOT get a Flu Vaccine 

JAMA Patient Pages: Seasonal Influenza Vaccination and the Influenza Vaccine

Continue the Discussion:

Join the COVID-19 discussion groups for healthcare professionals on MedShr:


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Figure 1: Flu-related population mortality rates for those in risk groups, from Public Health England Health Matters edition focussing on flu immunisation programme and COVID-19. Available at: (licensed under the Open Government Licence v3.0)

Figure 2: Safe ways of delivering the flu immunisation programme, from Public Health England Health Matters edition focussing on flu immunisation programme and COVID-19. Available at: (licensed under the Open Government Licence v3.0)

Article by Dr Ryan Broll, MedShr Open Editorial Team, Medical Education Fellow

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