Archiving notice

Since their introduction in early 2020, COVID-19 vaccines have saved at least 1.4 million lives in Europe alone. Worldwide, more than 12 billion doses of the vaccines have been administered as of March 2024, and between 14 and 20 million lives have been saved. In light of this success and the generally high vaccination uptake in most countries around the world, we have decided to discontinue updating of this wiki. The existing material may be of interest for archival reasons and will remain accessible, but not further updates will be undertaken. If you have any questions about the wiki or the Handbook, please contact the authors.

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Facts about COVID-19 vaccines

• Facts about COVID-19 vaccines
  • How do the vaccines work?
  • How many people have received the COVID-19 vaccines?
  • How safe are the vaccines?
  • How are vaccines being tested?
  • Efficacy of COVID-19 Vaccines
    • The Pfizer-BioNTech vaccine
    • The Moderna vaccine
    • The Oxford-AstraZeneca vaccine
    • The Janssen (Johnson and Johnson) vaccine
    • How to interpret the results
    • Points to note about interpretation
  • Effectiveness of COVID-19 Vaccines
  • What about "breakthrough infections"? Why do so many vaccinated people get infected?
  • What about waning immunity?
    • Basic components in the immune response to COVID-19
    • Immunity and COVID-19 variants
    • How do boosters help?
  • Facts against disinformation

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Vaccines provide a possible path out of the COVID-19 pandemic, and it is therefore fortunate that scientists have now developed several vaccines against COVID-19. Those vaccines have been found to be highly effective against the disease, and some provide a protection level of around 95% in controlled trials.

• For facts about the disease COVID-19 see our page here.
• Find out how successful vaccines have been in the past at our page here.
• Find out more about how the COVID-19 vaccines were developed at our page here.
• Find out more about COVID-19 vaccines for children at our page here.

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How do the vaccines work?

Here is a two-minute introduction by Dr Asher Williams at Cornell University about how the mRNA vaccines, by Pfizer/BioNTech and Moderna, work:

This video by Joe Hanson explains how vaccinations help protect the population (including those who are unable to be vaccinated, e.g., for medical reasons) by developing herd immunity:

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How many people have received the COVID-19 vaccines?

Our World in Data provides an interactive chart of how many people received a vaccine against COVID-19 around the world. The chart is embedded on this page and can be explored without leaving this page.

_{Source: Our World in Data.}

The European Centre for Disease Prevention and Control (ECDC) provides a vaccine tracker specifically for the European region. The ECDC Vaccine Tracker is based on data reported by EU/EEA countries to the European Surveillance System (TESSy) and is updated weekly. The tracker allows several ways of visualising data per country and also allows to download the data. The charts from the ECDC Vaccine Tracker can be accessed and explored by clicking on the excerpt from the tracker below.

Several other live trackers also allow you to get up-to-date statistics on vaccinations:

• The New York Times Coronovirus vaccine tracker (it also provides a lot of in-depth information regarding the different vaccines)
• The Bloomberg COVID-19 vaccine tracker.

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How safe are the vaccines?

We have a page dedicated to side effects of the vaccines.

Considerations about vaccine safety should also be assessed against the risks of the disease it protects us against. Our page on risk perception discusses this.

Several national and international regulatory agencies have examined the data and have authorized vaccines before they can be used:

• The U.S. Food and Drug Administration gave emergency approval to the Pfizer-BioNTech COVID-19 Vaccine in December 2020. This graph explains how emergency approval is granted (click for full size image):
  
  It shows that "emergency approval" requires a series of steps that ensure the safety of the vaccine notwithstanding the emergency:

For an EUA [emergency approval] to be issued for a vaccine, for which there is adequate manufacturing information to ensure quality and consistency, FDA must determine that the known and potential benefits outweigh the known and potential risks of the vaccine. An EUA request for a COVID-19 vaccine can be submitted to FDA based on a final analysis of a phase 3 clinical efficacy trial or an interim analysis of such trial, i.e., an analysis performed before the planned end of the trial once the data have met the pre-specified success criteria for the study’s primary efficacy endpoint.

• The World Health Organization (WHO) gave emergency validation to the Pfizer/BioNTech vaccine on 31 December 2020–-the first vaccine to receive validation since the outbreak began.
• More details about safety can be found at this Metafact page.
• A great resource about vaccine safety generally is the Vaccine Safety Net.
• National Geographic has a synopsis of the current state of the most promising vaccine developments
• The European Medicines Agency has a detailed page about the development, approval, and monitoring of COVID-19 vaccines.

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How are vaccines being tested?

Two key terms are used to describe the success of a vaccine: Efficacy refers to the degree to which a vaccine prevents a disease in controlled clinical trials. The numbers reported below therefore refer to efficacy. Effectiveness, by contrast, refers to the success of a vaccine in the real world, established by observational studies. A vaccine with high efficacy would be expected to also be highly effective in the real world, but the actually achieved effectiveness may differ from the controlled studies.

The Centre for Disease Control (CDC) provides a summary of vaccine testing procedures here. There are 3 test phases that any vaccine goes through before it is licenced:

• During Phase 1, a small number of people receive the trial vaccine. This phase mainly checks for adverse side effects, but also verifies the body's immune response.

• During Phase 2, the vaccine is administered to people who are similar (in terms of age and physical health) to the population for whom the new vaccine is intended.

• During Phase 3, the vaccine is given to thousands or tens of thousands of people. The emphasis here is on efficacy–-does the vaccine prevent illeness?–-and also safety. For all practical intents and purposes, Phase 3 is the most important part of testing because it tells us about safety and efficacy on a large scale.

If all phases are completed successfully, and the vaccine has been shown to be effective and without serious side effects, regulatory bodies approve the new vaccine for release.

Once a vaccine is approved and licensed, "testing" continues during actual use, a stage known as Phase 4. This phase consists of monitoring the side effects, if any, of the vaccine while it is being administered to the public.

We have an entire page devoted to the way in which the COVID-19 vaccines were developed, and why they could become available so quickly despite being thoroughly tested.

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Efficacy of COVID-19 Vaccines

Vaccinated individuals have a lower risk of contracting COVID-19 compared to unvaccinated individuals. For example, in one study the risk of contracting COVID was reduced by 95% in the group that received the COVID-19 vaccine compared to the control group. Vaccinated individuals also have a lower risk of developing severe symptoms if contracting the disease compared to unvaccinated individuals.

We summarize the method and results for the Phase 3 tests of the three main vaccines rolled out to date: Pfizer-BioNTech, Morderna, and Oxford-AstraZeneca. In all three studies, volunteer participants were randomly assigned either to a placebo control group that did not receive the vaccine or to a vaccine group that was administered the COVID-19 vaccine. Because the assignment was random, the only thing that differed between groups was whether or not people received the vaccine. In consequence, if we find a difference in the incidence of the disease, then we can be certain that it reflects the effect of the vaccine.

Altogether, the trials involved more than 85,000 participants from around the world.

The Pfizer-BioNTech vaccine

The Pfizer-BioNtech trial involved 43,448 individuals (older than 16) who were randomly assigned to two groups: One that received the vaccine and one that received a placebo. Ultimately, 18,556 people received two doses of the vaccine 21 days apart, and 18,530 received a placebo. (Some participants withdrew along the way, so the two numbers do not add up to the original 43,448.) The figure shows the trial design:

Once both doses of the vaccine (or placebo) were administered, the incidence of COVID-19 was monitored for both groups (i.e., around 37,000 people). The bottom line was that only 8 people in the vaccine group contracted the disease, compared to 162 in the placebo. In other words, the vaccine was 95% effective.. The next figure provides another snapshot of the data, by plotting the cumulative number of cases (severe cases are identified by solid plotting symbols) in both groups over time after the first dose (the inset is showing the same data but on a different time scale).

The divergence between the placebo and vaccination group is striking. Note that the time axis starts after the first dose, and that full protection would not be expected to kick in until 7 days after the second dose –- corresponding to day 28 in the graph.

The Moderna vaccine

The Moderna trial used a very similar design to the Pfizer-BioNTech trial. A total of 30,420 participants were randomly assigned to either the placebo or the vaccine group. Participants were again observed for around 90 days after receiving the second dose. Altogether, 269 cases were observed in the placebo group, compared to 19 in the vaccinated group. The efficacy of the vaccine was estimated to be around 93-94%.

The Oxford-AstraZeneca vaccine

The Oxford-Astrazeneca randomized 23,848 participants into the usual two groups, of whom 11,636 were included in the interim analysis reported in the paper. Vaccine efficacy was estimated to be 90% in a regime involving a low dose followed by a standard dose (it was 62% under a regime of two standard doses). These results are confirmed in the updated primary efficacy results which include data from 17,178 individuals and show an overall vaccine efficacy of 66·7%. after the second dose. In the participants who received two standard doses, after the second dose, efficacy was higher in those with a longer prime-boost interval (vaccine efficacy 81·3%) than in those with a short interval (vaccine efficacy 55·1%). This data supports the current UK policy of a 3-month interval between the 2 vaccine doses, whereby vaccination of a larger number of individuals with a single dose while vaccine supplies are limiting may provide better overall population protection than vaccinating half the number of individuals with two doses in the short term.

We are monitoring the latest updates on side effects associated with the Oxford-AstraZeneca vaccine on our side effects page.

The Janssen (Johnson and Johnson) vaccine

The Janssen vaccine has been approved for use in the US. Although, in early April 2021, the US CDC paused the roll-out of Janssen for a safety review, on 23 April 2021, the FDA and CDC expressed confidence that the vaccine is safe and effective in preventing COVID-19, and its use should be resumed (pdated 1st June 2021). However, one, and especially women under the age of 50, should be aware of the rare risk of blood clots with low platelets after vaccination.

On the 28th May 2021, the single-dose Janssen vaccine also approved for use in the UK by the medicines regulator (MHRA). In trials, the vaccine proved to be 85% effective in stopping severe illness from Covid-19, and met expected safety standards.

How to interpret the results

'The vaccines were shown to be 95%, or more, effective'. What exactly does "95% efficacy" mean? We explain this by considering the Pfizer-BioNTech clinical trial.

Results from the clinical trial in Phase 3 indicate that vaccine efficacy one week after the first dose was 52% (95% credible interval 29.5% to 68.4%), with 39 cases of COVID-19 in the vaccine group and 82 cases in the placebo group.

Seven or more days after the second dose, the participants were again examined for COVID-19 symptoms. In total, 162 people in the placebo group and 8 people in the vaccine group were found to have at least one of the COVID-19 specific symptoms, indicating they were sick from COVID-19. Thus, vaccine efficacy after the second injection was 95% (90.3% to 97.6%), with 8 covid-19 cases reported in the vaccine group and 162 cases in the placebo group.

To understand where the percentages come from, we work out that in the vaccine group, 154 people escaped the disease: We know that 162 would have likely gotten the disease without vaccination, and because only 8 did, we protected 154 people from the disease. This is what we call efficacy of the vaccine: 154 / 162 = 0.9506 = 95%. You can see a more detailed analysis (albeit with slightly changed numbers) in this New York Times article.

We can scale up those numbers to illustrate how effective the vaccine is. If we had vaccinated 1,000,000 people, only 430 would be expected to get sick. Out of 1,000,000 people who were not vaccinated, 8,760 would get sick from COVID-19. The difference between the two scenarios would be 8,760 - 430 = 8,330. This is the number of people who would be saved from getting sick with COVID-19 thanks to having been vaccinated. In total, this means that, in a population of 1,000,000, out of the 8,760 people who would get sick without vaccination, 8,330 would avoid the disease thanks to the vaccination. The effectiveness again is 8,330 / 8,760 = 0.9509 = 95%. (Note that in reality, the effectiveness of the vaccine may be lower than the efficacy observed in laboratory studies, as noted above, so these numbers are illustrative only.)

Points to note about interpretation

1. The efficacy of a Phase 3 clinical trial only estimates the protection provided by vaccination to the people who get vaccinated in a short period after vaccination. It is not meant to explore long-term effects ('duration of immunity'), as this is something that is evaluated once the vaccination begins in the general population.
2. The efficacy of a Phase 3 clinical trial only estimates the direct protection to the people who get vaccinated. This means that it cannot account for the people who are protected indirectly thanks to herd immunity. This means that the actual number of people who are protected thanks to vaccination directly and indirectly is even larger.
3. In case of only one injection of the Pfizer-BioNTech vaccine, 39 cases of COVID-19 were found in the vaccine group and 82 in the placebo group, leading to a 52% efficacy. This means that if people receive only one injection, half of the people who got vaccinated would be spared from the disease. Although not as impressive as the 95% of the two injections, saving half the population from getting sick would be still a huge accomplishment. However, it is not known at the moment how long the protection offered by a single shot lasts.
4. For more information on statistical literacy regarding health statistics, see: Gigerenzer, et. al. (2007)
5. The actual statistics used by some of the trials to compute efficacy were more complicated and sophisticated than the illustrative computations shown here.

The trials also recorded side effects from the vaccination, which we discuss in a separate dedicated page.

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Effectiveness of COVID-19 Vaccines

Data on the effectiveness of the vaccines in "real-life" situations are beginning to become available in early 2021.

Some of these results are from Israel, which has been leading the world in its vaccination drive. According to the Times of Israel, Pfizer's vaccine has been 92 percent effective, based on data from 163,000 people vaccinated by one healthcare service.

Only 31 out of those 163,000 (or 0.02%) were diagnosed with COVID-19 in their first 10 days of full-strength protection. In an equivalent sample of unvaccinated Israelis, that rate was 11 times higher, yielding an estimated efficiency of 92%. This number is gratifyingly close to the 95% efficacy observed in controlled trials.

On 10th May 2021, new evidence showed that vaccination is highly effective in protecting against death and hospitalisation from coronavirus (COVID-19). New Public Health England (PHE) analysis shows for the first time that individuals who receive a single dose of the AstraZeneca vaccine have approximately 80% lower risk of death with COVID-19 compared with unvaccinated individuals. The report also shows that, for those who have received the Pfizer-BioNTech vaccine, protection against death is approximately 80% after one dose and 97% after 2 doses (Vasileiou et al. 2021; AvonCAP)

Protection against infection has also been seen in healthcare workers, where a single dose of Pfizer vaccine provided more than 70% protection against both symptomatic and asymptomatic infection (Hall et al 2021), and in care home residents where a single dose of either Pfizer or AstraZeneca vaccines reduced the risk of infection by around 60% (Shroti et al, 2021). This suggests that vaccination has potential to reduce transmission; this was supported by a Scottish study that showed a 30% reduction in risk of infection in the household members of vaccinated compared to unvaccinated healthcare workers (Shah et al, 2021).

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What about "breakthrough infections"? Why do so many vaccinated people get infected?

Nearly 80% of all car occupants killed in recent traffic accidents in the U.K. were wearing a seatbelt. Only just over 20% of fatalities were not wearing a seat belt. So how come we are being told that seat belts save lives?

Seat belts save lives, and most people know that, which is why between 97% and 99% of British drivers wear seat belts. Now compare the two sets of numbers: the small share of drivers (1-3%) who don’t wear seat belts make up more than 20% of all fatalities—up to 20 times more than would be expected based on how few careless drivers there are.

Vaccines are like seat belts.

Vaccines reduce the likelihood of infection, hospitalization, and death from COVID-19. The reduction in deaths is particularly striking: As we show in the section on effectiveness, people who have received two doses of the Pfizer vaccine are 97% less likely to die from COVID-19 than unvaccinated people.

That’s great news.

But now imagine that everybody in a country is vaccinated (an optimistic assumption, although some countries come close; Portugal has vaccinated nearly 90% of its population). What proportion of people who are infected would have been vaccinated? What share of hospital patients would have been vaccinated? And what proportion of fatalities? The answer in all cases is 100%–in the same way that if everyone always wore a seat belt, all traffic fatalities would have been seat-belt wearers. But of course the total number of fatalities would be far lower than if no one wore a seat belt.

It doesn’t matter how safe seat belts or vaccines are—if everyone (or a large majority of the population) are protected, then the few failures of the protection—known as “breakthrough infections” in the case of vaccines—may seem like a large number. To illustrate with data provided by MIT in August 2021, only about two hundredths of a percent (0.02%) of all fully vaccinated people tested positive per week—but that is still 35,000 people, which will seem like a large number unless you consider that more than 164,000,000 vaccinated people did not get infected. Over time, as more people are vaccinated, the number of breakthrough infections will necessarily increase—but that does not mean the vaccines do not protect you.

A recent analysis by the New York Times of CDC data shows that the case rate among unvaccinated individuals is 6 times higher than among the vaccinated, and the death rate among the unvaccinated is 12 times greater. Whatever the absolute number of cases, your risk is far lower if you are vaccinated than if you are not.

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What about waning immunity?

The issue of waning immunity is an essential one, especially at the current time when many people worldwide have now received a COVID-19 vaccination.

Two components play a role here. First, the time that has passed since the last vaccination and second, whether there are new variants of the virus that have an influence on the effectiveness of the vaccine.

Basic components in the immune response to COVID-19

The immune system and the components involved are the subject of much more in-depth study, but the most important factors include:

Antibodies. You've probably already heard the term antibodies discussed in the context of vaccines, referring especially to the neutralising antibodies that are responsible for protecting us against infection or transmission. The production of these antibodies is induced by vaccination or infection. Studies have shown that the number of antibodies decline after a certain time after immunisation, so that the ability to protect us from infection decreases (Naaber et al., 2021). Fortunately, the body has more mechanisms at its disposal. For example, the long lasting memory B-cells. They help to induce renewed antibody protection, whenever our body gets into contact with the virus. Findings to date show that they last much longer than just a few months (Ciabattini et al., 2021).

T-cells. Additionally, there are white blood cells that we call T-cells. Their production is also induced by vaccination or infection. These T-cells are important in controlling the severity of disease we experience (Tan et al., 2021). This means that even if you may get an infection, the risk of hospitalisation or death due to COVID-19 decreases.

Antibody and T-cell counts increase again after booster vaccinations with mRNA vaccines such as Pfizer-BioNTech and Moderna. A large-scale study, in which a wide variety of vaccination regimens were tested, showed that it did not matter which vaccine was used for the prior vaccinations: as long as a mRNA vaccine was used for a booster shot, the protective ability was good (Munro et al., 2021).

Immunity and COVID-19 variants

Just as science evolves, so does the virus. The kind but also the count of antibodies needed to adequately protect against infection has unfortunately changed due to the emergence and rapid spread of the Delta and now the Omicron variant.

Already with the Delta variant, it has been shown that the effectiveness of two vaccinations is reduced compared to prior variants (Pouwels et al., 2021; Planas et al., 2021). In addition, with the waning immunity over time, the protection against infection is reduced (Tartof et al., 2021).

How do boosters help?

A booster vaccination increases the antibody count again and therefore decreases the risk of so-called "breakthrough infections" due to variants such as Delta (Demonbreun, 2021.

These booster vaccinations are also needed to further restore the ability of antibodies to neutralise the Omicron variant (GeurtsvanKessel, 2021). The increasing antibody count after the booster vaccination can induce an effective response against this new variant (Garcia-Beltran et al., 2022), even if it is just for a short time. Studies have shown that the antibody count declines again after a short while (Wilhelm et al., 2021)

But it is again the T-cells that give us hope here for continued protection: current studies have shown that the T-cell response, which is induced through vaccination, is effective across the variants–-including Omicron. This means that a booster vaccination is still very important to protect us against severe infections (GeurtsvanKessel, 2021).

In Israel, the effectiveness of a second booster vaccination is currently being tested, especially to discuss protection against the increasingly dominant Omicron variant. Some of the vaccine manufacturers are already testing the development of an adapted vaccine in parallel. The results of this and the development of the variants will show what further measures and vaccine recommendations will look like.

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Facts against disinformation

Unfortunately, misinformation about the COVID-19 vaccines has been circulating online. There are a number of reputable sources that counter disinformation:

• The Center for Disease Control has a page dedicated to the COVID-19 vaccines.
• The European Medicines Agency has a page dedicated to the COVID-19 vaccines.
• A rich resource regarding COVID-19 vaccination is the CDC website.
• The European Vaccination Information Portal has a fact page with many interesting links.

We have a separate page that is dedicated to addressing common myths about the COVID-19 vaccine.

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_{Page contributors: Stephan Lewandowsky, Teresa Gavaruzzi, Konstantinos Armaos, Mark Winfield, Laura Rivino, Jolanta Bernatoniene, Parichehr Samsrizi, Anahita Fathi}

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_{Note: text in green identifies latest update. (See last updated time stamp at top of page.)}

_{This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 964728 (JITSUVAX).}

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