Virus literacy for beginners: Influenza and Covid-19

In this article, we explore several questions that are being asked about what causes Covid-19, its relationship with ordinary “influenza” and where these viruses are thought to come from. 

What causes influenza?

Influenza is caused by a virus. There are four types of influenza virus; A, B, C and D.

There are then various subtypes of these four types. Human influenza A and B viruses are the cause of the seasonal epidemics of disease (known as the flu season) almost every winter. Influenza A viruses are the only influenza viruses known to cause flu pandemics, that is, global epidemics of flu disease.

A pandemic can occur when a new and very different influenza A virus emerges that both infects people and has the ability to spread efficiently between people.

Covid-19 is not an influenza virus. It is a coronavirus which is a type of virus. Coronaviruses (CoV) are a family of viruses including, but not limited to, SARS-CoV and MERS-CoV.

Why is Covid-19 different?

Covid-19 and influenza both cause respiratory symptoms and are primarily spread through small droplets from the nose or mouth. Covid-19 is caused by a single virus known as Severe Acute Respiratory Syndrome Coronavirus 2, or SARS-CoV-2. This is a new virus and therefore people do not have immunity.

Covid-19 generally seems to lead to more severe disease than seasonal flu strains partly due to this lack of immunity. There is no vaccine and such a vaccine may be many months away. The situation surrounding Covid-19 is changing rapidly; the most recent mortality rate is 3.4% while that of the flu is less than 1%, according to the WHO. The WHO believes that containment of Covid-19 is possible with active involvement of countries by contact tracing in order to prevent infections and save lives.

How do new viral strains emerge?

New influenza viral strains constantly arise from two mechanisms that alter the genetic code of a virus. These mechanisms are known as genetic drift and genetic shift. Genetic drifts are small changes in genetic material called point-mutations which can occur in a single virus strain. Genetic shifts are when the genetic material of different strains of influenza viruses mix and form new combinations. 

Another possible way a new strain could emerge?

Human infections with avian and other zoonotic influenza viruses, though rare, have been reported infrequently. These infections are primarily acquired through direct contact of humans with infected animals or contaminated environments, but usually do not result in efficient transmission of these viruses between people.

It is possible that when pigs become infected with both human and avian influenza virus these interactions can act as an intermediate step in combining the genetic material of avian, human, and swine influenza. The influenza genetic strands reassort while in the pig to form new combinations of avian, swine, and human strands. In turn, these new influenza viruses, which could possibly infect humans, have features not previously seen by the human immune system. This means that a human lacks immunity and that illness, sometimes severe, may occur.

How does the flu vaccine work?

The seasonal flu vaccine protects against the influenza viruses that research indicates will be most common during the upcoming season.

Most flu vaccines protect against four different flu viruses (“quadrivalent”); two influenza A viruses, and two influenza B viruses. Getting the flu vaccine can protect against flu viruses that are similar to the viruses used to make the vaccine. But the flu vaccines will not protect against infection and illness caused by other viruses, such as Covid-19, that also can cause influenza-like symptoms.

Why do new influenza strains and other viruses seem to originate in South East Asia?

There are a number of theories as to why new strains originate in South East Asia but no clear evidence. Some of the theories are:

• Southeast Asia contains more than half of the global population. Larger populations should sustain larger virus populations, and in the absence of other reasons, they should contribute to a larger proportion of strains that happen to spread globally.


• Birth rates have historically been higher in South East Asia than in most temperate populations. High birth rates influence the susceptibility of the population due to increased percentage of non-immune individuals resulting in decreased immunity to a virus strain and thereby allowing for genetic change.


• Strong trade connections between Europe, North America and Australasia exports new viruses to other countries.