A guide to viruses, vaccines and Polymerase Chain Reaction (PCR)

Since the onset of COVID-19 in January 2020, there have been lots of conversations and news articles about viruses, how they survive (and thrive), and the role vaccines can play in helping protect against them.

Viruses have been around for millennia and encompass everything from the common cold and chickenpox to hepatitis and meningitis.

But what are viruses, how can they be used to create vaccines and what role does Polymerase Chain Reaction (PCR) play in detecting viruses. Jackie from our Philip Harris Technical Support team covers all this and more.

What is a virus?

Put simply, a virus is:

An infective agent that typically consists of a nucleic acid molecule in a protein coat, that is able to multiply only within the living cells of a host.

The sole aspiration of a virus, and their entire reason for existing, is to reproduce. In order to do this, a virus needs a ‘living’ host.  When a virus finds a suitable host cell, it will replicate itself, and its offspring will spread to new cells, and new hosts and so on.

The way a virus is made up will affect the way it is transmitted, its ability to spread and ultimately, its ability to survive.

Viruses can be transmitted between hosts in a variety of different ways including:

• By the touching of an infected surface. For example, some viruses including COVID-19 can live on the surfaces of objects for a long time, so if a person comes into contact with an infected surface, the virus will be transmitted.
• Inhaling minute droplets from another person’s cough or sneeze.
• Ingesting infected food or water.
• Transmitted though insects, who can carry the virus from one person to another.

Once a virus has found a host cell inside the body it will start to replicate which could cause the infected person to start showing symptoms of the virus.

How can viruses be used to create vaccines?

Not all viruses are bad, however. And some viruses can even be used to help us. The most common example is when viruses are used in vaccines to help eliminate diseases.

There are several different types of vaccines that can be used to protect against the diseases caused by viruses and some of them use the virus itself, or at least some form of the virus.

The most common types of vaccine are:

Inactivated vaccines – uses a version of the virus that causes the disease, but the scientists kill or inactivate the virus, to prevent it from replicating when it enters a host.  This type of vaccine isn’t as efficient as a live vaccine, and they often require a booster shot to achieve beneficial immunity.

Live/ attenuated vaccines – These use a weakened form of the virus that causes the disease they are protecting against.  Usually a single dose, or maybe 2 is all that is requires to a give a lifetime protection against the disease.  There are however some downsides to this type of vaccine and they are not advised for people with weakened immune systems, as although they are using a weakened form of the virus, they are still introducing a live version.

What role does Polymerase Chain Reaction (PCR) play?

Polymerase Chain Reaction or PCR tests have become a key tool in the global fight against COVID-19 and the effort to control the spread and outbreak globally. No doubt by now, you are all familiar with the term PCR. But what does it actually mean?

To detect the presence of a virus, a PCR test can be carried out.  PCR is a technique that is used in microbiology to make copies or replicate DNA.  It can also be used to detect genetic material from a virus.

The PCR test will be able to show the presence of a virus, providing the test is carried out whilst the person is infected.  Although due to the nature of the PCR process, it can also allow fragments of the virus to be detected even if the person is no longer infected.

The PCR test is done in 3 steps.

Step 1: Collecting the sample.  This is usually carried out using a swab which is inserted into the nose (this is where respiratory material is usually found).  There are different types of nasal swabs which can be used, and the type used depends on where the virus is most likely going to be located.  After sample collection, it is vital the swab is placed in a sealed, labelled, sterile vessel ready for being sent to a lab for testing.

Step 2: The lab will perform an extraction process on the sample to collect the genetic material from the virus.

Step 3: The PCR process is carried out on the extracted genetic material by a PCR Machine.  This typically involves 3 steps: heating, temperature reduction and heating again.

The first heating stage denatures the DNA of the target sample, unwinding the helix and breaking the bonds. This leaves scientists with a single strand of DNA.

The temperature reduction stage allows the primer, a short strand of DNA that attaches to each end of the target sample, to start working. The primer binds to the strand of DNA and provides a starting place for the replication.

The final heating stage allows elongation to begin and, in this step, the tag polymerase (an enzyme) binds to the end of the primer and using the single strand DNA as the template, incorporates the nucleotides.