What Is the Influenza Virus?

The influenza virus has several subtypes and it's been considered that, in many cases, the necessary epidemiological surveillance hasn't been carried out as it should have been done in the regions where the virus has appeared.
What Is the Influenza Virus?

Last update: 26 August, 2019

The influenza virus is a severe respiratory disease that can lead to complications causing hospitalization and even death if not treated in time.

These particular subtypes of viruses belong to the Orthomyxoviridae family and were the cause of various severe pandemics. Stay with us and let’s learn a little about its history.

What is the influenza virus?

During World War I, the appearance of a flu pandemic hit the world by storm. It’s currently known as influenza or the Spanish flu. The number of deaths caused by this epidemic was quite devastating.

According to data from the Chilean Journal of Infectology, it’s estimated that the Spanish flu was responsible for 20 to 40 million deaths worldwide in less than a year. It’s been calculated that a fifth of the world’s population had become infected.

Its origin is directly related to a disease in swine o r farm animals , as described by an inspector of the US animal industry office in 1918. However, this claim remained unvalidated until 1928, when Dr. Richard Shope, of the Department of Comparative Pathology of the Rockefeller Institute in Princeton, became the first to describe the viral etiology of swine influenza.

Subtypes of the influenza virus

The influenza virus circulates in various parts of the world. In China, for example, the origin of the H2N2 influenza virus caused a pandemic in 1957.

More recently, epidemics of the avian H5N1 influenza virus and the H9N2 virus have infected people in Hong Kong. Therefore, it’s essential to know the subtypes of influenza that currently exist:

H5N1 Influenza

In November of 1997, 18 confirmed cases of H5N1 occurred in Hong Kong, followed by other epidemics such as the avian H5N1 virus in chicken farms.

Scientists couldn’t control the outbreak until 2001. Even then, scientists confirmed that they identified a new case in 2003. Experts consider H5N1 to be a highly aggressive strain with significant epidemiological repercussions.

Influenza H9N2 virus

After the H5N1 epidemic occurred in Hong Kong, epidemiological surveillance was reinforced in the province where the illness appeared. This led to the isolation of nine human samples of the influenza H9N2 virus, between July and September 1998.

Then in March 1999, H9N2 was isolated in two other Hong Kong patients, but the disease was mild, and transmission between humans was less likely.

Influenza H1N2 virus

At the beginning of 2002, a new influenza H1N2 virus appeared, which was isolated from patients with influenza in England. This particular virus mainly affected young children. It originated from the genetic rearrangement of circulating influenza A H1N1 and H3N2 strains.

H7N7 Influenza

At the end of February 2003, a fatal case appeared, in which 82 people in their majority developed symptoms of conjunctivitis, only seven had respiratory symptoms. Therefore, this strain became known as the influenza H7N7 virus.

In Asia, pigs, ducks, and man coexist frequently. The transfer of viruses between species increases the virus’ virulence.


The production of an effective anti-influenza vaccine has presented several challenges for the pharmaceutical industries working on it. Currently, it’s difficult to control the possible occurrence of adverse effects.

Therefore, every year, different countries make an effort to maintain an epidemiological alert, study the antigenic variations of circulating viruses, and, thus, develop appropriate vaccines. ACIP experts advise people in the following circumstances to get vaccinated:

  • People over 65 years old.
  • Children over six months old.
  • Children who are older than six months or adults with chronic metabolic diseases, including diabetes, kidney failure, hemoglobinopathies, or immunosuppression.
  • People who live or are in continuous contact with high-risk patients.
  • Personnel working with the elderly or managing patients considered as high risk.

Vaccine effectiveness

The effectiveness of vaccinations may vary from one person to another. The effectiveness of the influenza vaccine changes year after year, depending on the degree of antigenic similarity between vaccine strains (chosen nine to ten months before the epidemic period) and the strains circulating at the time.

In a study published in the Journal of the Faculty of Medicine at the National University of Colombia, the efficacy was higher for the H3N2 subtype, due to the genetic identity of this vaccine with the strain causing the outbreak. On the other hand, the vaccine is less effective in preventing the disease; even though it reduces the severity of major symptoms.

Symptoms of influenza

This respiratory disease is widely contagious, so parents should be especially attentive to the symptoms that the child presents. We can characterize the symptoms of the virus as appearing abruptly and unexpectedly; these may include:

  • Fever.
  • Cough.
  • Sore throat.
  • Nasal mucus or stuffy nose.
  • Muscle and body aches.
  • Headaches.
  • Fatigue.

The influenza virus can appear suddenly, and it can be really dangerous and contagious, so parents should stay alert and consult a specialist promptly before the symptoms appear.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Barrientos, R. S. V., & Reyes-Terán, G. (2007). El virus de la influenza. Neumología y cirugía de tórax66(S1), 12-14.

  • Acuña, L. (2004). Influenza: Historia y amenazas. Revista chilena de infectología21(2), 162-164. http://dx.doi.org/10.4067/S0716-10182004000200012.

  • Córdoba, M. A. V. (1999). Influenza: agente etiológico, manifestaciones, diagnóstico, prevención y control. Revista de la Facultad de Medicina47(2), 82-88.

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.