Vaccine effectiveness in the 'real world'

Globally, tens of millions of people have now been vaccinated against COVID-19. With this, our understanding of how effective vaccines are at reducing the burden of and protecting against the virus in the ‘real world’ continues to grow.

A key success factor for the vaccines is to eliminate serious disease, hospitalisation and death from COVID-19. Vaccines that demonstrate real-world effectiveness against these factors are of critical importance in managing the pandemic and easing the restrictions currently in place around the world. 

What does vaccine effectiveness mean?

How is vaccine effectiveness assessed?

Vaccine effectiveness is assessed by analysing real-world evidence. Real-world evidence helps people better understand if a medicine or vaccine is effective in day-to-day clinical practice, as opposed to ‘efficacy’ which is determined in controlled clinical trials.

Clinical trials provide a good indication of how well a medicine or vaccine works and its safety profile, but when this is observed in the real-world setting, a wider view of its impact can be monitored.

In combination with clinical trial data, real-world evidence helps inform the bigger picture – how effective a medicine is in larger, more varied populations, in various healthcare settings and over an extended period.

By using this evidence, scientists, regulators and governments can monitor the important contribution a medicine or vaccine is making to society.

Specifically, real-world evidence for COVID-19 vaccines is:

  • Helping governments, health providers and communities understand how well vaccines are working across demographics.
  • Informing and supporting strategies to increase vaccine uptake to protect lives and reduce hospitalisations.
  • Informing longer-term use of vaccines and adjustments that might give additional protection in terms of vaccine boosters, dosing or combination treatments.

What determines vaccine effectiveness?

How well the vaccine works to protect individuals

As the number of people vaccinated against COVID-19 grows, we are seeing important real-world evidence emerge, which is helping to understand vaccine effectiveness:

  • Against different disease severities – from reducing deaths and hospital admissions to prevention of milder forms of the disease
  • In reducing rates of infection across the population
  • In preventing transmission and spread of the virus
  • In different populations, including age, race, ethnicity, and those living with underlying health conditions, such as asthma, diabetes and HIV

Level of immune response sustained over time

COVID-19 vaccines work by priming the body’s natural defences so that they are ready to act quickly and effectively if you later encounter the virus. This immune response, or immunogenicity, can be measured to help us understand how effective vaccines are.

As vaccination programmes continue, the results of immunogenicity studies will help us understand the type of immune response that vaccines trigger and how long it lasts. They will also help inform the right dose of a vaccine, as well as if and when booster vaccinations may be required.

Ease of use for healthcare communities

Logistical aspects and ease of use are crucial to ensure broad access to and uptake of a vaccine. This may include how easily a vaccine can be transported and stored (ideally by using a cold chain that is already in place for other vaccines), how stable the vaccine is and for how long, and how easily it can administered within existing health care settings e.g. no need for specially trained medical staff.

Broad access to the vaccine and uptake

To change the course of the pandemic, COVID-19 vaccines need to be available globally and accessible to all who need them. Global partnerships with scientists, governments, multilateral organisations and manufacturers are currently in place to ensure development capacity and supply.

In terms of uptake, governments are responsible for administering vaccinations in line with their vaccination programmes.

The vaccine’s risk: benefit profile

No medicine or vaccine is ever entirely risk-free. When evaluating COVID-19 vaccines, regulators have considered both the benefits and risks before allowing them to be administered to the public. Safety monitoring has also been paramount, and for vaccines in particular, the standard of testing is higher than for most other medicines, as vaccines are generally given to healthy people.

Safety monitoring procedures are in place for all COVID-19 vaccines, with data thorough scrutinised by pharmaceutical companies, regulators, scientific and medical experts. Real-world evidence is helping to inform both effectiveness and safety, with the World Health Organization (WHO) affirming that the benefits of vaccination outweigh the risk of potential side effects.1

Informing future strategies

Evidence of real-world effectiveness, combined with data from ongoing clinical trials, is improving our knowledge of how vaccines are working while informing future strategies to protect against COVID-19.

We continue to learn more about the virus too – different variants, prevalence, and transmissibility, and in time we will be able to predict these changes and start to plan for them.

To overcome the disease, it is likely that many different vaccines will be needed to support long-term immunity and for protection against new variants as they arise. The potential of combining different vaccines, known as ‘heterologous boosting’, is currently being researched and may improve protection and help make immunisation programmes more flexible in the future.


1. World Health Organization (WHO). Statement on the benefits of vaccines against COVID-19. Available at Last accessed May 2021.


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Veeva ID: Z4-37095
Date of Preparation: August 2021