The development of a Covid-19 vaccine (effective against the SARS-CoV-2 virus) has been a significant global achievement. Usually, it would take as long as a decade to develop a vaccine and have it released into the market for human use. In just a year, a Covid-19 vaccine was developed, trialled and approved for use, with the world’s first dose being administered in the UK on 8 December 2020. Since then, billions of doses have been administered worldwide. By May 2021, there were more than 12 different vaccines approved for use globally with more in development, and by May 2022 more than 11 billion doses had been administered globally.

How does the vaccine work? 

Pathogens such as bacteria, fungi and viruses are microorganisms that cause disease. When a person encounters a harmful pathogen, they are susceptible to the associated disease. The body has natural defence mechanisms to protect itself from harm. Skin, mucous, hair cells and cilia (tiny hair cells in the lungs) all prevent the entry of a pathogen into the body. However, if the pathogen breaches these defences, a person’s immunity will launch a defence in an effort to destroy it.

To launch this defence, the body must have already been exposed to the pathogen, at which point it would begin to develop antibodies. In future exposures, the body will recognise the virus as an invader and trigger the pathogen specific antibodies and memory cells to respond immediately. People have thousands of antibodies, but each is specific to an antigen. Each antibody can identify a specific antigen and fight it off.

However, when the antigen is new (as was the case with SARS-Cov-2) the body does not have the necessary antibodies or memory cells required to fight it off. In the case of a virulent pathogen, such as SARS-CoV-2, the body may not be able to develop antibodies quickly enough to prevent infection, something that was seen with the high morbidity and mortality rates associated with the Covid-19 infection.

The advantage of a vaccine is that the body does not have to be exposed to the pathogen in order to develop immunity. This immunity decreases the risk of developing the disease and any adverse health outcomes associated with it. Instead, vaccines contain a weakened or inactive antigen from the pathogen, which is used to trigger an immune response when it enters the body. In this way, the necessary antibodies and memory cells needed to launch a defence against the pathogen are developed. The weakened antigen does not cause the disease, but still encourages the body to manufacture antibodies and memory cells. Some vaccines may require a booster dose to strengthen the response of the antibodies and memory cells.

A person vaccinated against Covid-19 will be protected against severe disease, but not everyone can be vaccinated. This is because of underlying, immunocompromising health conditions or severe allergies to a vaccine component. Those who can’t be vaccinated can still be protected if they live among others who are vaccinated. When a point is reached when most people in a community are vaccinated or have antibodies because they have had the disease, the pathogen will have difficulty circulating because most of the people it encounters are immune. Consequently, where large numbers of people are vaccinated, the less likely it will be for unvaccinated people to be exposed to the harmful pathogens. This is referred to as “herd immunity” and is a key benefit of vaccination programmes.

Side effects

It is common for individuals to experience mild side effects after vaccination. This is because the immune system identifies the foreign pathogen and instructs the body to react in certain ways; for example, by increasing blood flow so that more immune cells can circulate, or by raising the body’s temperature in order to kill the virus. These are signs that the body is building protection. Mild-to-moderate side effects (ie pain at the injection site, low-grade fever, fatigue, headache or muscle aches) are normal, differ from person to person and are not a cause for alarm. These side effects usually subside a day or two after the vaccination is administered. It is important to also note that not experiencing side effects doesn’t mean that the vaccine has been ineffective. Individuals respond differently.

After vaccination, it takes around 14 days for the body to build immunity against SARS-CoV-2 virus. It is possible that a person could be infected with SARS-CoV-2 just before or after vaccination and still get sick with Covid-19 because the vaccine has not yet started to provide protection.

Benefit of the vaccine

Intelligence gained during clinical trials has provided evidence of the safety and efficacy of the Covid-19 vaccines. From around the 14th day post-vaccination, individuals are increasingly protected against severe illness and death from Covid-19. There are several Covid-19 vaccines available, and some may require a second dose (known as a booster dose) to boost the level of antibodies and memory cells in the body. The protection provided is especially important for high-risk groups (ie the elderly and front-line healthcare workers) who otherwise would remain vulnerable to the severe health effects of Covid-19. The vaccine is seen as a way out of the pandemic because it reduces the transmission of the virus from person to person. No vaccine is 100 percent effective at preventing infection and not everybody has been vaccinated, making it necessary to also continue practicing non-pharmaceutical measures of protection (ie such as social distancing and wearing masks).


The Access to Covid-19 Tools (ACT) Accelerator is a ground-breaking global collaboration to accelerate the development, production and equitable access to Covid-19 tests, treatments, and vaccines. COVAX is co-led by Gavi, CEPI and the WHO and aims to accelerate the development and manufacture of Covid-19 vaccines and guarantee fair and equitable access for every country in the world. Global equitable access to a vaccine, particularly protecting healthcare workers and those most-at-risk, is the only way to mitigate the public health and economic impacts of the pandemic. The first COVAX deliveries of vaccines were to Ghana in Feb 2020; since then vaccines have been distributed to more than 100 economies, increasing the coverage in developing economies. COVAX has shifted focus from discovery of vaccines to equitable supply and delivery of the available vaccines. It is working towards the WHO aim to achieved 70% global vaccination of adults by the end of 2022.