Vaccines have been a critical tool in the fight against the virus, and as COVID-19 response actors, it is essential to understand how COVID-19 vaccines work to effectively communicate accurate information and address public concerns. This resource aims to provide a comprehensive overview of the COVID-19 vaccines, including their efficacy, safety, and the challenges associated with their deployment. By equipping response actors with this knowledge, we hope to support vaccination efforts and promote public health amidst the ongoing pandemic.

Response actors need to be able to explain vaccines and related terms in clear, concise, and actionable language through trusted sources. This communication may be easier with the aid of videos or illustrative articles. The video below from the Vaccine Knowledge Project is useful to visualize how vaccines harness the natural activity of our immune system in a way that’s easy to understand and explains key technical terms. WHO’s “Vaccine Explained” and “Science in 5” series also provides a structured set of illustrated articles on how vaccines work, how they are developed and the different types of vaccines available.

Lastly, the Johns Hopkins COVID-19 Training Initiative has developed three training modules on vaccines for health care workers, including how to communicate about them, which can be highly beneficial for response actors seeking to build trust in vaccination.

Vaccines are preventative substances - they are used to prevent infection, serious illness, and death. Vaccines teach the body’s immune system to recognise, and therefore be able to stop, the cause of a specific disease before it can make someone ill. Vaccines are usually given by injection, but they can also be taken by mouth or sprayed into the nose.

The immune system is the part of the body that fights infection and disease. The active ingredient in different types of vaccines is made from deactivated or non-infectious organisms or a part of the organism (e.g. a protein) that you are trying to train your body’s immune system to stop. The part of the organism is called an antigen and can’t cause disease. When your immune system is introduced to these harmless antigens in very small amounts through vaccination, it learns what the cause of an illness (e.g., a specific virus or bacteria) ‘looks like’ so that your immune system builds specially targeted antibodies ready to fight off any future infection with this type of organism. This means that if you are fully vaccinated and get infected with the actual organism months, or for some vaccines years later, your immune system will recognise it and destroy the invading organisms before it causes disease.

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Millions of people are alive today due to vaccination. Vaccines have helped reduce the global burden of tetanus, diphtheria, and measles and can now combat epidemics such as Ebola and cholera. The first vaccines were introduced in the 18th Century. Today, most countries have routine vaccinations, often known as ‘routine immunisation’ programmes, which are given to people at different stages of their life, usually starting in childhood. Research estimates that routine vaccination against 10 common diseases (such as Hep B, rotavirus, measles, and rubella) has prevented 37 million deaths between 2000 and 2019.

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The COVID-19 vaccines have been developed and approved through a rapid, yet safe, process due to emergency funding and unprecedented levels of global coordination and collaboration. This has been possible for a number of reasons. In part it is because COVID-19 is not the first coronavirus to threaten health in recent years. From the Severe Acute Respiratory Syndrome (SARS) coronavirus in 2003 and Middle East Respiratory Syndrome (MERS-CoV) coronavirus in 2012, scientists have had a head start in understanding the properties of coronavirus SARS-CoV-2. The unique structure of these viruses has enabled the development of a vaccine in record time through leveraging knowledge and technology platforms. There has also been unprecedented investment and research into vaccine development due to the urgency to stop COVID-19 pandemic.

There are various methods and approaches that are used to assess the effectiveness of vaccines. All of the globally authorised vaccines that are most advanced in their research, trials, and production use four different approaches, and here Wellcome Trust explains their differences. Whilst this Johns Hopkins University resource explains the clinical trial process for vaccines.

All COVID-19 vaccines approved by the World Health Organization (WHO) undergo the same review and regulatory process, with safety and efficacy review boards composed of independent scientists not involved in vaccine development.

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As mentioned above COVID-19 vaccines must go under strict reviews. There is an external panel of experts, that analyse the results from clinical trials, evidence of the outcome of the vaccine on the disease, risk factors and other information. Thereafter the experts are equipped to recommend whether the vaccine should be used or not.

Once the vaccine is out, there are multiple systems in place to monitor that vaccine’s safety and effectiveness.

Vaccine safety monitoring, as with other medicines, happens at a national and global level. There are vaccine adverse event reporting systems; these systems collect and analyse the reports of adverse events that happen after the vaccine has been administered. Anyone can submit a report. If after thorough research, the adverse events are linked to the vaccines; the regulating agencies may decide to pause the administration of the vaccine and make sure that the problem does not outweigh the vaccine benefit. Take a look at this video by Wellcome, where they explain in more detail the systems in place to make sure that the vaccine you receive is safe.

COVID-19 vaccines have been tested in large, randomized controlled trials that include people from many different ages, all sexes, different ethnicities and those with a known medical condition.

… during pregnancy?

People who are pregnant or were recently pregnant are more likely to get severely sick from COVID-19, compared to those who are not. Severe disease includes the need to go to the hospital, intensive care or an illness that results in death. Additionally, people who get COVID-19 during pregnancy have a higher risk of preterm birth, still birth and other pregnancy complications. Evidence continues to show that the COVID-19 vaccination during pregnancy is safe and effective, to both the pregnant person and the baby and is not associated with fertility problems.

… if you want to become pregnant?

There is no evidence that the COVID-19 vaccine has any effect on fertility, regardless of gender. While some people have reported changes to their menstrual cycle after being vaccinated, this has been identified as a temporary side effect and has no impact on fertility. The changes were similar for the different types of COVID-19 vaccines.

…while breastfeeding?

It is safe to get the COVID-19 vaccine if you are breastfeeding. Some recent reports have shown that people who got a Pfizer or Moderna vaccine (mRNA COVID-19 vaccine) have antibodies in their breastmilk, which could help protect their babies. But more information is necessary to understand to what extent these antibodies protect the baby.

…for children?

In contrast to older adults, SARS-CoV-2 tends to cause mild disease and fewer deaths in children and adolescents. For this reason the benefit of vaccinating a child is not as big as the one of vaccinating an adult. When the pandemic started, with the original SARS-CoV-2 virus, children were a minority of reported global cases. But this significantly changed during the Omicron variant, where there was an increase in the number of children and adolescents affected. Nevertheless, several studies have shown that COVID-19 vaccines are safe and effective for children to have them. WHO recommends that countries consider the individual and population benefits of vaccinating children depending on each country’s context. For example, in the UK, the NHS recommends that the vaccine is offered to all children between the ages of 12 and 15 and some aged 5-11. In the US, the CDC recommends COVID-19 vaccines for everyone ages 6 months and older and for everyone ages 5 years and older if eligible.

There are many words and phrases used to explain COVID-19 and vaccines. It’s important that key terms are explained so that people understand them.

Immunity

Immunity to a specific infection means that if someone gets that infection, their body’s defence (immune) system can recognise the infection and stop it before it causes disease. For example, if someone is fully vaccinated against COVID-19, it means that if they get the coronavirus SARS-CoV-2, their immune system will recognise it and destroy it, so it cannot cause severe COVID-19. Some level of immunity can also be gained from having the disease. This is often called natural immunity.

Susceptible/Unsusceptible individuals

Viruses, such as SARS-CoV-2, exist to reproduce and replicate. However, viruses need a susceptible host, someone who is not immune, in order to replicate. If a vaccinated, uninfected person is exposed to an infected person, the vaccinated uninfected person is not a susceptible host. As a result, the virus will have a reduced ability to spread. If you have enough people within a community that are vaccinated and less susceptible, the virus dies out.

Herd Immunity

Herd immunity is the community protection that is created when a high percentage of the community is vaccinated, which provides a protective barrier to those who have not been vaccinated, or unable to be vaccinated (e.g., immune-compromised). Currently, the world has not reached herd immunity against COVID-19. This video – How does herd immunity work? – provides a practical explanation.

For COVID-19, the coverage necessary to reach herd immunity remains unknown. However, data suggest the coverage level is between 70% and 85% of the population being fully vaccinated. As we have not yet reached this level, we cannot rely on herd immunity now. This makes it critical to promote vaccine uptake and continue to encourage preventive behaviours such as hand hygiene, appropriate mask-wearing, physical distancing, and covering coughs and sneezes. It is also essential that countries are equipped with the diagnostics to detect and test, track and trace contacts, facilitate the quarantining of cases and provision of medical care to those who are seriously ill.

Vaccine Efficacy

Vaccine efficacy is how effective a vaccine is at preventing infection and disease-related morbidity and mortality. Many different endpoints (comparison between vaccine candidates and within differing populations) are used in vaccine research to describe vaccine efficacy. In the case of SARS-CoV-2, an efficacious vaccine might prevent infection, disease, hospitalisation, intensive care unit admission, death, or transmission as per the figure below.

It is important to mention that vaccine efficacy and effectiveness are not the same thing. Effectiveness refers to how well a vaccine works in the real world; by contrast, efficacy refers to how well it performs under ideal circumstances.

COVID-19 vaccines approved by WHO are extremely effective in preventing death and severe disease. This means that if you are fully vaccinated, your chances of infection and hospitalisation are reduced by at least 80%. Check out GAVI’s article explaining the effectiveness of COVID-19 vaccines.

Experts are still learning how effective COVID-19 vaccines are at stopping transmission of SARS-CoV-2. Recent research suggests that those given a single vaccine dose can reduce household transmission of the virus by up to half (vaccinated people are between 38% and 49% are less likely to pass on than unvaccinated people).

Efficacy is also dependent on how many doses of a vaccine are needed. For a single-dose vaccine, our immune system will take a few weeks after vaccination to mount an antibody response to the vaccine. For a two-dose vaccine, both doses are needed to provide the best level of protection possible. The duration of protection from vaccines is currently unknown and we are less clear about how long natural immunity lasts.

Effectiveness of a vaccine may also vary with the spread of new variants in different countries. We discuss more about the variants of the virus in the next section.

Booster doses

A booster vaccine is an additional does or doses of the vaccine given after the protection provided by the original shot has begun to decrease over time. Like other vaccines, it is necessary to get additional doses to have a better protection and be safer. This is particularly important if you suffer from another disease or are a health worker or carer. According to the UK Health Security Agency, evidence suggests that most people will have around 65% protection against serious illness after five months, and 52% after one year.

This information is based on research conducted in July 2022, when the dominant strain was Omicron BA.5. Depending on the country you are on, you may be offered a dual vaccine or Omicron booster (also called bivalent booster), this means that in addition to protecting you against the original SARS-Cov-2, they also protect you from other variants like Omicron.

It is important, as for all vaccines, to be continuously monitored. As we have learned, there are many factors that affect how a vaccine work, depending on each person but also on the evolution of the virus. The original vaccines were developed based on the SARS-CoV-2 that was present in 2020, but there have been some changes to this virus. We discuss more about the variants of the virus in the next section.

Variants & Mutations

Like all viruses, SARS-CoV-2, is mutating (changing in small ways over time). This is a normal evolutionary process and true of all viruses and other living things. Even with these mutations, it is still the same SARS-CoV-2 virus. The mutations that have arisen so far have meant that the virus can pass more easily between people, cause more or less severe illness, or slightly different symptoms. Globally, several variants have emerged, and these variants differ in their related morbidity and mortality. WHO, in joint collaboration with their global partners have assessed the evolution of SARS-CoV-2 and have characterised specific Variants of Interest (VOIs) and Variants of Concern (VOCs) in order to systematically prioritise the global monitoring and research, which is critical to the pandemic response. The simple, easy-to-say labels for the VOI and VOC can be found here. Proactive efforts are already in place in real-time to revise the vaccine to tackle vaccine variants and mutations. One advantage of the current vaccines is that they can be reconfigured quickly, and therefore will play an important role in responding to the variants.

As mentioned above, there have been new vaccines developed that focus on particular variants, like Omicron.

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You may have noticed that not every country started vaccinating the same group of people, for example in some countries, the first to get vaccinated were older adults, in other it was frontline workers.

In an ideal situation everyone would be vaccinated at the same time, unfortunately there is normally less vaccines than people in the world. Factors such as the risk of severe disease, how exposed you are to that risk are considered. For example, your country might have decided to vaccinate first people with existing diseases and elderly because they have a higher risk of illness, dying or having severe disease. Similarly, other countries prioritized the vaccination of healthcare workers and essential workers given their risk of exposure to the virus. Other factors as equity also need to be considered, for example disadvantaged sociodemographic subpopulations at higher risk of severe COVID-19. Because of the nature of the SARS-CoV-2 virus, children, who can be a vulnerable population, were not the most severely affected at the beginning of the pandemic, for these reasons they were not prioritized. Nevertheless, as time has passed research has shown that children should be vaccinated and that the vaccine is safe for them, particularly if they have an underlying health condition.

These decisions are the Ministry of Health of each country informed by Regulatory Agencies or Committees, both national and international. The WHO shares recommendations for countries to follow. For more information on how people were prioritized for vaccination a look at WHO access and allocation explainer.

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Originally written by: Jenny Lamb (LSHTM – Hygiene Hub)

Revised and reviewed by Ana Bolio (LSHTM – Vaccine Confidence Project) and Jenny Lamb (LSHTM – Hygiene Hub)