Influenza: Epidemiology, Physiology, Key symptoms, and Treatment

On average, about 8% of the U.S. population gets sick from flu each season, ranging between 3% and 11%, depending on the season suggested by A 2018 CDC study published in Clinical Infectious Diseases. Influenza can be seasonal or unpredictable Pandemic (COVID-19).

Flu of variable severity occurring in the U.S. result in significant morbidity and mortality. Seasonal flu occurs annually and is termed an epidemic. 

What is influenza?

Influenza is commonly known as flu. The causative agent of Flu is Influenza Virus. Influenza typically infects the respiratory system because it is an airborne disease that occurs by breathing in the polluted or infected air. All parts of the respiratory system, starting from the Nose, Throat, and lungs, are infected by flu.

What are the types of influenza?

There are mainly four types of influenza. Type A, Type B, Type C, and Type D. According to World Health Organisation (WHO):

• Influenza A viruses are further classified into subtypes according to the combinations of hemagglutinin (H.A.) and neuraminidase (N.A.), the proteins on the virus’s surface. Currently circulating in humans are subtype A(H1N1) and A(H3N2) influenza viruses. The A(H1N1) is also written as A(H1N1)pdm09 as it caused the pandemic in 2009 and subsequently replaced the seasonal influenza A(H1N1) virus, which had circulated before 2009. Only influenza type A viruses are known to have caused pandemics.
• Influenza B viruses are not classified into subtypes but can be broken down into lineages. Currently, circulating influenza type B viruses belong to either B/Yamagata or B/Victoria lineage.
• Influenza C virus is detected less frequently and usually causes mild infections, thus does not present public health importance.
• Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

What is the Epidemiology of flu?

All age groups are susceptible; however, some are at more risk than others.

Pregnant women, children under the age of 59 months, the elderly, people with chronic medical conditions (such as chronic cardiac, pulmonary, renal, metabolic, liver, or hematologic diseases), and people with immunosuppressive conditions (such as HIV/AIDS, receiving chemotherapy or steroids, or having cancer) are all at higher risk of developing severe illness or complications when infected.

Due to increased patient exposure, healthcare personnel is at significant risk of contracting the flu and spreading it to vulnerable people in particular.

Flu season spreads quickly and easily in crowded settings, such as nursing homes and schools.

Seasonal epidemics primarily occur in the winter in temperate climes, although flu may circulate all year round in tropical areas, leading to more sporadic outbreaks.

The incubation period, also known as the transition phase, is the period between infection and disease and is typically between one and four days.

Etiology/Cause of Influenza?

The majority of cases of the flu are spread by tiny airborne droplets that are released when a person coughs or sneezes. If you come in contact with something that has the flu virus on it and then touch your mouth, nose, or eyes, you may fall sick.

People frequently confuse flu with cold. Although they are unrelated, you could have some of the same symptoms. Many people experience colds throughout the year. Runs the contrary, the flu often only strikes people periodically.

You may occasionally contract a virus that causes diarrhea or vomiting. This is referred to as the “stomach flu” by some. Because this virus is not the flu, this term is misleading.

Physiology of Influenza?

Flu is an acute illness that affects the trachea and upper respiratory system, causing inflammation there.

In most healthy people, the condition is self-limited after seven to ten days of acute symptoms.

The viral syndrome, which includes a high fever, coryza, and bodily aches, is caused by the immunological response to the viral infection and the interferon response.

High-risk populations (Groups with co-morbidity) are more likely to experience severe side effects, including hemorrhagic bronchitis, primary viral pneumonia, secondary bacterial pneumonia, and even death. These severe side effects can appear as soon as 48 hours after the onset of symptoms.

Beginning at the time of incubation and reaching a peak, on average, within 48 hours, the virus replicates in the upper and lower respiratory passages.

Key Symptoms of Influenza Virus?

Flu symptoms range from mild to severe, and in extreme cases, they may lead to death. Flu has a sudden onset of symptoms. Below listed symptoms are key indicators of flu:

• Blocked or Runny nose
• Cough
• Sore Throat`
• Headache
• Body aches
• Fatigue
• Fever/Chills (Note: Fever may or may not occur to everyone suffering from influenza)

Prevention and Treatment of Flu?

The most effective defense against the flu is vaccination.

The flu vaccine guards against infection and helps people avoid serious illnesses. People require a new vaccine each year to ensure their protection against the flu viruses, which are continually evolving. 

Even when the circulating viruses do not precisely match the vaccine viruses, persons in good health can still benefit from the influenza vaccine.

The flu vaccine may be less successful in avoiding illness among the elderly, but it does lessen the severity of the illness and the likelihood of complications and fatalities.

For those who live with or provide care for those who are at high risk for flu complications, vaccination is especially crucial.

Pregnant women at any stage of pregnancy, children between the ages of 6 months and five years, adults over 65, people with chronic medical conditions, and healthcare workers are among the groups WHO advises getting vaccinated annually.

Mild flu improves on its own without medical intervention. Resting, drinking plenty of fluids, especially water, taking paracetamol to lower temperature and pain, and utilizing decongestant medications are all ways to treat the symptoms.

People with severe flu symptoms might need to visit a hospital.

Antiviral drugs can treat or stop the spread of influenza, particularly during outbreaks in healthcare facilities like hospitals and residential institutions. Treatment for influenza A and B is possible with neuraminidase inhibitors. Two drugs from the adamantanes antiviral family are efficient against influenza A but not influenza B. Recently, there hasn’t been much of a problem with the virus developing resistance to these antivirals, but that could change at any time.

Colds and the flu, which are viral diseases rather than bacterial ones, should not be treated with antibiotics.