Influenza and COVID-19: Some Important Background Information

Seasonal influenza is a highly contagious airborne disease that manifests as an acute febrile illness with variable degrees of systemic symptoms, ranging from mild fatigue to respiratory failure and death. Influenza causes significant loss of work days, human suffering, and mortality. This year, concerns about influenza overlap with the growing threat from a second wave of COVID-19.

According to Medscape, influenza causes as many as 650,000 deaths annually worldwide. In England, the following chart, taken from a Public Health England publication on flu, shows the number of deaths from influenza across all ages in the past five seasons.

According to the CDC, COVID-19 and seasonal influenza share many common symptoms, including the following:

• Fever or feeling feverish/chills
• Cough
• Shortness of breath or difficulty breathing
• Fatigue
• Sore throat
• Runny or stuffy nose
• Muscle pain or body aches
• Headache
• Vomiting and diarrhea (more common in children than adults)

Unlike with influenza, individuals with COVID-19 may develop a change in or loss of taste or smell (anosmia).

Other key differences between COVID-19 and influenza include the following:

• Persons with influenza typically develop symptoms more quickly after infection than those with COVID-19. Influenza symptoms usually develop 1-4 days after infection. COVID-19 symptoms typically develop 5 days after infection, although it can be as early as 2 days or as late as 14 days after infection.
• For both influenza and COVID-19, spreading infection is possible for at least 1 day before experiencing any symptoms. Those with COVID-19 may be contagious for a longer period than if they had influenza. Older children and adults with influenza appear to be most contagious during the first 3-4 days of their illness, and many remain contagious for about 7 days. The exact length of time someone with COVID-19 can spread infection is still under investigation.
• Overall, the risk for complications in healthy children is higher for influenza than for COVID-19. However, infants and children with underlying medical conditions are at increased risk for both influenza and COVID-19.
• Both COVID-19 and influenza are predominantly spread person-to-person, between those who are in close contact. Both infections are spread mainly by droplets made when people cough, sneeze, or talk. However, it is possible for individuals to get infected from either virus by physical human contact or by touching a surface or object that has virus on it and then touching their mouth, nose, or eyes.

Public health officials have warned that influenza vaccination is particularly important this year, owing to concerns about a potentially deadly confluence of the seasonal flu and COVID-19.

A CDC analysis previously reiterated the importance of vaccinating pregnant individuals regardless of trimester. The CDC and others recommend vaccination as long as influenza viruses are circulating, even in January or later. Influenza vaccine provides reasonable protection against immunized strains. The vaccination becomes effective 10-14 days after administration.

In the USA, guidance about individuals recommended for the flu vaccine are available here: Seasonal Influenza (Flu): Information for Health Professionals. In the U.K., further details on eligible groups are available via this link.

In relation to USA guidance, according to Medscape, Guidelines from the IDSA recommend the following:

• During influenza activity, clinicians should test for influenza in high-risk patients with symptoms that resemble influenza, pneumonia, or nonspecific respiratory illness if the testing result will influence clinical management.
• Clinicians should also test for influenza in patients who present with acute onset of respiratory symptoms with or without fever and either exacerbation of chronic medical conditions or known complications of influenza (eg, pneumonia) if the result will influence clinical management.
• Nasopharyngeal specimens should be collected over other upper respiratory tract specimens to increase detections of influenza viruses. If nasopharyngeal specimens are not available, nasal and throat swab specimens should be collected and combined together for testing over single specimens from either site. Mid-turbinate nasal swab specimens should be collected over throat swab specimens.
• Clinicians should not collect or routinely test for influenza specimens from nonrespiratory sites, such as blood, plasma, serum, cerebrospinal fluid, urine, and stool.
• Clinicians should use rapid molecular assays (ie, nucleic acid amplification tests) over rapid influenza diagnostic tests in outpatients to improve detection of influenza virus infection.
• Clinicians should use reverse-transcription polymerase chain reaction or other molecular assays over other influenza tests in hospitalized patients to improve detection of influenza virus infection.

Treatment guidelines for influenza vary by country, while those for COVID-19 are still being developed and refined on the basis of ongoing studies.