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Explainer: What is a vaccine and why are they so important?

Let’s get back to basics.

Image: Shutterstock

AN EFFECTIVE VACCINE for Covid-19 is within our sights now, with the HSE expecting to start rolling out its programme in January. 

It is not clear which Covid-19 vaccine will be the first to be administered in Ireland, though the Pfizer/BioNTech vaccine – which has already been rolled out in the UK – is first on the list for the European Medicines Agency to assess for approval. 

People have a lot of questions about how these vaccines were developed, how they work and how safe they are. At TheJournal.ie we’ll be working over the coming weeks to answer as many of those questions as we can. 

We get into the more minute detail on Covid-19 vaccines in our Q&A series, but here we’ll go back to the basics.

What does a vaccine do?

While other medications treat or cure diseases, vaccines are designed to prevent them.

A vaccine stimulates your immune system to produce antibodies like you would if you were exposed to a virus or disease. The aim is to teach your body to recognise the virus if you do actually catch it at some stage and to attack it before it makes you sick.

Our immune systems are designed to remember exposure to viruses or bacteria so, depending on the vaccine, once people have received one or more doses, they can remain protected against disease for years, or even a lifetime. 

This cute video from PBS explains it in more detail:

Source: PBS/YouTube

Why do we have vaccines?

Aside from the obvious reason – to protect the individual who gets the vaccine from disease – there are wider impacts on communities and populations.

When a person gets vaccinated against a disease, their risk of infection is also reduced and this makes them far less likely to transmit the disease to other people.

The more people in a community who receive a vaccination, the less chance there is that a virus will spread through that community, potentially making some people very ill.

In some cases people may not be able to receive a particular vaccination due to other serious health conditions and these people can be better protected if a large number of people in their community have been vaccinated. 

‘Herd immunity’, a term used widely in the debate about Covid-19 over the last ten months, can only be achieved when a high proportion of the population is vaccinated.

Mass vaccination makes it difficult for diseases to spread because there are fewer potential hosts for the virus. 

How are they developed and tested?

Every vaccine has to go through testing before it can apply for regulatory approval.

In pre-clinical trials, an experimental vaccine is first tested in animals to evaluate its safety and potential to prevent disease. Then the developer will move to clinical trials, with three phases to work through before they apply for approval.

In Phase 1, the vaccine is given to a small number of people to assess safety for human use, to confirm it generates an immune response and to test dosage levels.

In Phase 2, the vaccine is given to hundreds of people. They are monitored for any side effects – mainly the more common side effects would be picked up at this stage.

At this stage developers also collect data on the efficacy of the vaccine in preventing disease, but numbers are too small for a clear picture. A proportion of the volunteers in this phase will receive a placebo.

In Phase 3, the vaccine is given to thousands, or tens of thousands of volunteers. Again, a proportion will receive a placebo and data from the groups is compared. This phase gives a clearer picture of how effective the vaccine is in eliciting an immune response and the appropriate dosage.

With a large – and more diverse – pool of volunteers, this phase also aims to catch less common side effects and examine the prevalence of side effects overall.

Once the results of these clinical trials are available, reviews of efficacy, safety and manufacturing are carried out before approval by regulatory authorities. 

Monitoring of vaccines continues after they are rolled out to detect any unexpected adverse effects and to further assess the effectiveness. 

What kinds of vaccines are there?

There are vaccines available to protect against at least 20 diseases such as diphtheria, tetanus, pertussis, influenza and measles. According to the World Health Organization, these vaccines save the lives of up to three million people each year.

Not all vaccines may be needed in your country, but you may need to get some before you travel to certain countries. 

In terms of vaccine development for Covid-19, there are several different potential vaccines. These include:

  • Inactivated or weakened vaccine viruses. These use a form of the virus that has been weakened so it can no longer cause disease but still generates an immune response because your system can still identify it as a threat. 
  • Protein-based vaccines. These vaccines use harmless fragments of proteins that mimic the virus to generate an immune response.
  • Viral vector vaccines use another virus that has been genetically engineered so that it cannot cause disease, but produces coronavirus proteins to generate an immune response.
  • Messenger RNA vaccines use genetically engineered RNA to generate a protein that prompts an immune response. This is a new vaccine approach and the Pfizer vaccine is the first mRNA vaccine to ever get approval in any country.

How does the regulatory process work?


The European Medicines Agency (EMA) is the regulatory body for vaccines used in the European Union.

Vaccines can only be approved by the EMA if they comply with all requirements of quality, safety and efficacy set out in the EU pharmaceutical legislation.

Developers are required to go through the steps outlined above, with three clinical trial phases, before they can seek approval. Developers submit results of all testing and investigations to the regulatory authorities in Europe. This is part of what is known as a marketing authorisation application.

The vaccine can only be approved after scientific evaluation has demonstrated their overall benefits outweigh their risks. 

The vaccine’s benefits in protecting people must be far greater than any side effect or potential risks. 

The EMA has said it ensures scientific experts evaluating medicines do not have any financial or other interests that could affect their impartiality. 

Once the EMA gives its opinion, the Europeam Commission reviews this opinion and grants EU-wide marketing authorisation. Then national authorities make their own decisions on the introduction and rollout of the vaccine in their countries. 

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Vaccine development for Covid-19 is being fast-tracked due to the public health emergency.

The EMA has set up a multidisciplinary Covid-19 task force, bringing together experts across the European medicines regulatory network to ensure a fast and coordinated response to the pandemic. 

Regulators like the European Medicines Agency have been providing early scientific advice as well as guidance in methods and study designs to help developers generate robust data.

Companies involved in development have also taken various approaches to reducing timelines such as using bigger teams of researchers and running clinical trial phases or studies in parallel. 

Instead of waiting until all of the clinical trials have been fully completed to assess their data, regulators have completed rolling reviews of Covid-19 vaccine candidates all the way through the development as the data becomes available. This essentially means much of their work is already done by the time a developer seeks approval. 

Source: EMA

In order to maximise transparency in its regulatory activities on treatments and vaccines for Covid-19, the EMA is shortening its standard publishing timeframes. It is also publishing information it does not normally publish for other medicines. 

When an evaluation of a Covid-19 vaccine candidate is complete, the EMA has the option of recommending a conditional marketing authorisation. This is a type of approval for medicines or vaccines that address unmet medical needs and in particular those that are to be used in emergency situations such as public health threats. 

This approval will only be granted where the benefit of immediate availability outweigh the risk of less comprehensive data than normally required. Conditional marketing authorisations are valid for one year and can be renewed annually but the holder is required to complete specific obligations (ongoing or new studies) to provide more comprehensive data. 

Do you have questions about the Covid-19 vaccine?

Email answers@thejournal.ie and we’ll do our best to get through as many as we can over the coming weeks.

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