Similarities and difference between Influenza (Flu) and COVID-19

Both influenza (flu) and COVID-19 are contagious respiratory diseases, with some of the symptoms very similar, but they are caused by different viruses: COVID-19 by SARS-CoV-2 and flu by influenza viruses. A lot is still unknown about COVID-19, but it seems as if it spreads more easily than influenza and may cause more serious illnesses in some people. It may have a longer incubation period and can be contagious for longer.

Will COVID-19 and influenza be present simultaneously?

Although it is uncertain, it is most likely that SARS-CoV-2 and influenza viruses will circulate simultaneously during the traditional influenza season. Furthermore, although less common, it is possible to contract both infections at the same time. Both may result in serious illness, hospitalization or death. As humankind was naïve to SARS-CoV-2 at the beginning of 2020, but had been exposed to many influenza viruses with possible antibody cross-protection, COVID-19 might be more deadly than influenza at this stage of the pandemic.

Is influenza vaccination indicated this season?

Vaccination against influenza is more important this season than ever before. It will reduce the risk of serious illness, hospitalization and death from influenza and simultaneously conserve potentially scarce health care resources. It is still unknown whether infection with influenza may impact the risk of infection with and disease progression of COVID-19.

Furthermore, some association was described between the use of the flu vaccine and protection against severe disease from COVID-19 – a potential extra benefit of influenza vaccination during this pandemic.

Although wearing a mask and social distancing may be partially protective against respiratory viruses, the best way to prevent influenza is for everyone 6 months and older to be vaccinated each year. Globally influenza activity remains at lower levels than expected and in the southern hemisphere is at inter-seasonal level. However, the response to the COVID-19 pandemic lead to reduced influenza surveillance and/or reporting activities in many countries. The activity during the coming season cannot be predicted. The exact time of onset of the season is also not known and to allow for the two-week period for antibody development, influenza vaccination should be administered as soon as it becomes available.

Counsel the patient about the risk of side effects after influenza vaccination (local reactions, such as redness, pain, or swelling at the injection site, and systemic reactions, including fever, chills, headache, and body aches). Influenza vaccination does not cause respiratory symptoms such as cough or shortness of breath. Such symptoms or a fever that does not resolve within 72 hours of vaccination without the use of antipyretics, should prompt the patient to contact their health care provider. COVID-19 should be ruled out.

Influenza vaccination after or during COVID-19 disease

There are no data to inform optimal timing of influenza vaccination in persons with COVID-19 or who are recovering from COVID-19 with regards to influenza vaccine effectiveness.

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For people who are sick with COVID-19 and who are already in a medical setting (e.g. in a hospital or other health care setting), flu vaccination should be deferred until they are no longer acutely ill.

Some medication used for treating COVID-19 may have immunosuppressing effects causing less optimal antibody production in response to influenza vaccination. The ideal time for vaccination after discontinuation of the medication is not known yet. These persons might also be at increased risk for severe illness due to influenza as a result of immunosuppression. Timing of vaccination for these individuals should be guided by the individual’s underlying risk of medical complications due to influenza, and the degree of influenza circulation in the local community.

Influenza and COVID-19 vaccinations

Investigations are under way for a future combined vaccine, but currently separate injections are required.

Information on the simultaneous administration of COVID-19 vaccines (none of which currently are live virus vaccines) with other vaccines are sparse. The CDC advises a minimum interval of 14 days before or after administration of other vaccines. This should not prohibit vaccination at any time should the benefits outweigh the potential unknown risks of coadministration. If the 14-day recommended interval is not adhered to there is no need to repeat any doses of the implicated vaccines.

Actions while waiting for diagnostic test results during the influenza season

Non-hospitalized persons with respiratory symptoms should self-isolate at home for the prescribed period even if testing negative for both viruses. Persons at high risk for complications from influenza should use oseltamivir as soon as possible while waiting for test results.

During the influenza season hospitalized patients with suspected influenza should start empiric oseltamivir treatment as soon as possible regardless of illness duration, without waiting for influenza testing results.

Influenza antiviral medications have no activity against SARS-CoV-2 viruses, nor do they interact with medications used for treatment of COVID-19 patients. Patients with a SARS-CoV-2 and influenza co-infection with a high risk for serious influenza complications should receive influenza antiviral treatment.