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Coinfection Risks

In winter, coronavirus and influenza circulate together. Simultaneous infection increases the risk of severe illness and death.

After examining 5,493 patients who fell ill with coronavirus or flu from January to April 2020, scientists from the National Public Health Infectious Diseases Service of England found that 1% of the sick people (58 people) had a coinfection.

Coinfected patients had a 2-fold higher risk of death than patients with COVID-19 alone. The most increased mortality is among the elderly. The risk of admission to the intensive care unit or death was 6 times higher among coinfected patients.

At the same time, scientists have found that the flu reduces the risk of contracting SARS-CoV-2.

Influenza Impact on The Risk 0f Contracting Coronavirus

Among patients infected with influenza, the risk of contracting coronavirus was 58% lower.

The likely reason is that the body produces interferon in response to the first viral infection. The interferon response of the innate immune system resists the second viral infection.

The reproduction rate of SARS-CoV-2 is lower than that of influenza. So if the infections start simultaneously, the coronavirus is suppressed. However, if influenza infection occurs after infection with SARS-CoV-2, coinfection will be detected.

Interestingly, the flu significantly reduced the chance of contracting the coronavirus only in adults over 19 and the elderly. However, in children and young adults under 19, there was no association between a positive influenza test and a positive SARS-CoV-2 test.

The study is detailed in the article “Is It Possible to Get Coronavirus and Flu at The Same Time“. The study was published in the International Journal of Epidemiology.

Influenza Promotes Coronavirus Infection of Lung Cells but Protects Heart Cells From Coronavirus

Coronavirus can lead to cardiovascular damage. Some patients with COVID-19 have elevated troponin levels, a marker of heart damage, and impaired cardiac function. Cardiovascular complications associated with COVID-19 include myocarditis, acute myocardial infarction, heart failure, and arrhythmias.

ACE2, the SARS-CoV-2 receptor, is expressed in many body tissues, which allows the coronavirus to infect cells in the lungs and the heart. In addition, the influenza virus can infect heart tissue and cause cardiac fibrosis and electrical dysfunction.

American scientists have shown that the influenza virus changes the expression of ACE2 and the ability of ACE2 to bind to the coronavirus spike protein. The influenza virus increases the expression of the coronavirus ACE2 receptor in the lungs and heart but affects the ability of the coronavirus to infect cells in different ways:

  • ACE2 in the lungs. ACE2 has no enzymatic activity in influenza-infected lung epithelial cells but can bind to the coronavirus spike protein. The influenza virus enhances the binding of the coronavirus spike protein to the ACE2 receptor, facilitating the entry and replication of the coronavirus. The influenza virus increases inflammation and tissue damage.
  • ACE2 in the heart. In the heart’s muscle cells, the influenza virus stimulates the enzymatic activity of ACE2 but suppresses the replication of the coronavirus. Suppression of replication is facilitated by increased production of nitric oxide NO2, which depends on the enzymatic activity of ACE2. Nitric oxide has antimicrobial activity against various pathogens, including DNA and RNA viruses.

Details of the study are in the article “Influenza Virus Interferes with Coronavirus’ Ability to Infect Lungs and Heart“. The study is published in the Cell journal.

Flu Vaccination Reduces Severe COVID-19 Risk

Influenza vaccines enhance the function of innate immune NK cells, natural killer cells that recognize and kill infected cells in the absence of antibodies.

  • A study by Italian scientists showed that flu vaccination reduces mortality from COVID-19. The flu vaccine activated the innate immune system, so the body was tuned to a quick antiviral response when confronted with the coronavirus.
  • A study by American scientists showed that flu vaccination over the past 1, 2, and 5 years reduces the incidence of coronavirus infection.
  • Dutch and German scientists confirmed that hospital employees vaccinated against influenza were less likely to become infected with SARS-CoV-2. The flu vaccine stimulated immune cells and improved cytokine responses.
  • Swiss and Brazilian scientists found that COVID-19 patients who recently received a flu vaccine were 8% less likely to need intensive care, 18% less likely to need invasive respiratory support, and 17% less likely to die.

For more information on the protective effect of non-coronavirus vaccines, see “Why non-coronavirus vaccines are effective against SARS-CoV-2“.

Findings

The flu reduces the risk of contracting the coronavirus. However, in the case of coinfection with coronavirus and influenza, the risk of admission to the intensive care unit or death increases by 6 times.

For early detection of coinfection, testing for influenza should be performed along with testing for SARS-CoV-2. Influenza vaccination is necessary to prevent serious illness, as well as early administration of antiviral drugs if coinfection is detected or suspected.

The flu shot may protect the heart by upregulating the expression of ACE2, an enzymatic activity. In addition, the flu vaccine may protect the lungs by enhancing the interferon response, which reduces the severity of SARS-CoV-2 in the lungs.

The flu shot can help cure a lung infection in coronavirus patients without worrying about heart dysfunction. Drugs enhancing ACE2 enzymatic activity and nitric oxide boosters may also help treat lung infections caused by influenza and coronavirus.

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