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FAQs on COVID-19 Vaccines

We answer some of the frequent questions on the different COVID-19 vaccines

Last update: January 10, 2023

Photo: Thirdman / Pexels

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On this page we will try to answer the questions you may have regarding these vaccines, mainly those approved in the European Union. The initial list of questions will be gradually expanded. Due to the constant emergence of new scientific data, it is recommended to check the date of the last update of each answer.


> General questions

> About the side effects

> About the global vaccination efforts

> About child vaccination

> About Treatments

1. General questions

Which are the most advanced vaccines against COVID-19?

The following table shows the vaccines against COVID-19 which are on phase III of clinical trials or have finished it.

Which of the approved vaccines are showing greater effectiveness?

mRNA-based vaccines (Pfizer-BioNTech, Moderna) are showing slightly greater effectiveness over time compared with adenovirus-based vaccines (AstraZeneca-Oxford, Janssen), especially against symptomatic infection but also against hospitalization, accordingto US and UK data. This is partly because the former induces higher antibody titers than the latter. Among the mRNA vaccines, Moderna seems to protect slightly better over time than Pfizer (e.g., this study in the US and this one in Catalonia ). This could be because the former contains more mRNA than the latter.

All vaccines have been shown to be effective as booster doses, regardless of the primary schedule, but mRNA vaccines induce a higher level of antibodies, which would increase their ability to protect against the Omicron variant .

Last updated on January 28, 2022.

What are the differences between the vaccines currently used?

Among the COVID-19 vaccines already approved or still in development, we can distinguish different types according to the technology they use. On one hand, there are the "classic vaccines" that use whole inactivated virus or viral proteins. On the other hand, there are those using more innovative technologies to introduce a gene sequence with the instructions so our body can produce a given viral protein. This genetic information can be introduced directly into the cell (messenger RNA vaccines) or via a viral vector that infects the cell but does not replicate. mRNA vaccines include the already approved Pfizer-BioNTech and Moderna vaccines. Viral vector vaccines include the Oxford/AstraZeneca vaccine, the Janssen (Johnson & Johnson) vaccine and the Sputnik vaccine by the Gamaleya Institute. All of them have been tested in people to prove their safety and efficacy throughout the three phases of clinical trials.

Pfizer / BioNTech and Moderna: messenger RNA

Both Pfizer / BioNTech and Moderna vaccines are based on mRNA. These last-generation vaccines provide our cells with the genetic sequence that codes for the Spike (S) protein of SARS-CoV-2. In other words, the genetic material contained in these vaccines, which is synthesised in the laboratory, provide our cells with the necessary instructions to produce the S protein. Once the S protein is produced, our immune system recognises it as foreign and generates S-specific antibodies and T cells which will help us clear the virus more rapidly in case of infection.

Oxford / AstraZeneca and Johnson & Johnson (Janssen): viral vector

These vaccines use a harmless virus to introduce into our cells the instructions needed to produce the SARS-CoV-2 S protein. This harmless virus (usually an adenovirus) is called a viral vector and has been modified in the laboratory so it can infect our cells but cannot replicate.

The Oxford / AstraZeneca vaccine usesf a modified chimpanzee adenovirus while the Janssen vaccine uses a modified human adenovirus called Ad26.

The viral vector infects our cells and delivers the instructions so that our own cells produce the S protein. Our immune system will then recognise this protein as foreign and generate Spike-specific antibodies and T cells.

Last updated on November 24th 2022.

Recording date: 26-27 November 2020. 

How do the new mRNA vaccines work?

What messenger RNA vaccines do is introduce into our cells a gene sequence, synthetised in the laboratory, that codes the Spike (S) protein of SARS-CoV-2. This allows our cells to make copies of the S protein, which will induce an immune response. This means that, when we get infected by the coronavirus, we already have antibodies (humoral immunity) and T cells (cellular immunity) ready to recognise and clear the virus.

Being so new, are they safe?

Yes. We are, without a doubt, before a most promising and innovative technology in the field of vaccines, but not unknown. Although it has not been approved for vaccines in humans before, it is the result of more than a decade of advances in the biomedical field. Furthermore, its safety has been tested throughout all three phases of clinical trials and, later on, on millions of people who have received the vaccine over the last three years.

Can they give us the disease?

No, because they do not introduce the attenuated virus into the body, but genetic material. Furthermore, this messenger RNA involves the synthesis of a protein of the virus, but not of the entire virus. What can happen is that we have some side effects the day after having been vaccinated, such as pain in the arm, headache or fatigue. This is normal, it is a sign that our immune system is responding.

Do they modify our DNA?

Not at all because messenger RNA does not access the nucleus of our cells, so it cannot be incorporated into our DNA. Furthermore, the mRNA is degraded within days.

Do they have advantages over other vaccines?

Yes, they are easier to design and do not involve the handling of infectious material during development.

Recording date: 26-27 November 2020.

Can we trust vaccines that have been developed so quickly?

COVID-19 vaccines have been developed in record time thanks to important technological advances and experience gained with the SARS and MERS coronaviruses. However, this does not imply that the process was not rigorous and that the usual steps were not followed. Clinical trials have three phases: the first one consists precisely in confirming the safety of the drug, a fundamental aspect that is also confirmed in phases II and III of the trial, which involve the participation of thousands of people.

Therefore, all approved vaccines have previously completed the three phases established in the clinical trials without any major severe adverse event reported. A vaccine is authorised when the benefits of vaccination largely outweigh the risks..

One of the main unknowns due to the speed at which clinical trials were carried out is how long will these vaccines protect.All data show that protection against infection and symptoms declines 4 to 6 months, although protection against severe disease and death remains high. For this reason, and with the spread of Omicron (which further reduces protection offered by the original vaccine), booster doses after 5-6 months are recommended, particularly for the most vulnerable populations (people aged over 60 or with underlying health conditions).

Last updated on November 23, 2022. 

Recording date: 26-27 November 2020.

Are commercialized vaccines subject to surveillance mechanisms?

Of course. This is what we call phase IV trials. In other words, from the moment vaccines are authorized to be distributed and jump to the market, their safety and effectiveness are continuously assessed. It is a phase as strict as the previous that besides monitoring the current vaccines, it will contribute to improving the development of future vaccines.

Updated on 18 December, 2020.

Recording date: 26-27 November, 2020.

How long does vaccine-induced immunity last?

This has been one of the main questions since the first vaccines were approved.

Duration of immunity may differ depending on the type of vaccine. Waning of protection seems to be faster for inactivated virus vaccines (Sinopharm, Sinovac) than for adenoviral-vector vaccines (Janssen and AstraZeneca-Oxford) or for mRNA vaccines (Pfizer and Moderna).

Follow-up studies in vaccinated cohorts point to a waning of antibody levels over the 6 months following vaccination . In addition, omicron has a greater capacity to evade anibodies generated by the original vaccine, .

For these reasons, a first booster was recommended for everyone after the arrival of Omicron (early 2022), and now a second booster is being offered (preferably with Omicron-adapted vaccines), starting with the elderly and most vulnerable populations.

It is important to point out that, despite the decline in antibody levels, memory B cells and T cells induced by two doses of vaccine remain stable, which explains why protection against severe disease and death remains high for all vaccines, even against Omicron and its subvariants.

Last updated on November 23 2022.

Publishing date: 16 December 2021.


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Can I get COVID-19 if I have been vaccinated?

Yes. All authorised vaccines were shown to be highly effective against disease caused by pre-Omicron variants (over 90% for the Alpha variant and over 70% for the Delta variant), but even then, no vaccine was 100% effective. With the arrival of Omicron, the number of vaccinated people (with two or even three doses) that have been infected increased considerably. But it is important to emphasize that the vast majority of vaccinated persons (especially those who have received all three doses) are protected against severe disease and death , regardless of the variant.

Updated on 23 November, 2022.

Can I test positive or infect someone because I have been vaccinated?

No. None of the vaccines inoculate the whole, active virus. In the case of vaccines based on mRNA (Pfizer, Moderna), or viral vector (AstraZeneca, Janssen), the only thing that is expressed is a SARS-CoV-2 viral protein (the Spike protein), which cannot infect cells or replicate.

The Sinopharm or Sinovac vaccines consist of the entire virus, but it is inactivated and unable to infect or replicate. Therefore, although we can test positive in a serological test (which measures anti-Spike antibodies in the blood), it is not possible to test positive in a diagnostic test (PCR or rapid test) - which detects the presence of the virus in the respiratory tract- as a result of getting vaccinated.

Updated on 17 June, 2021.

If I have been vaccinated, can I spread the virus?

The clinical trials were designed to test whether vaccines prevent COVID-19 symptoms, not whether they prevent infection. However, it soon became clear that vaccination also reduced viral transmission. Although their impact on transmission is lower for Omicron, vaccines continue to reduce viral transmission by lowering the probability of getting infected in the first place.

Last updated on November 23, 2022.

Publishing date: 16 December 2021.

If I have already had the disease, do I need to get the vaccine?

The vast majority of people who have been infected develop immunity (virus-specific antibodies, B and T cells) that are estimated to protect them against severe disease likely for several years. However, the antibody levels among infected people are very heterogeneous and decay with time. In addition, “natural” immunity seems to provide less protection against the Omicron variant. This is why it is important to get the vaccine even if you have been infected. Numerous studies show that hybrid immunity (infection plus vaccination) is more robust and long-lasting than vaccination or infection alone.

Last updated on November 23, 2022.

Recording date: 26-27 November 2020.

Is it possible to combine two different vaccines?

Many studies show that combining different vaccines (heterologous regimen) is safe and induces equal or even superior immune responses than using two doses of the same vaccine (homologous regimen).

The same applies to boosters. A UK study confirms that any of the currently approved vaccines can serve as a booster, regardless of the initial regime, although overall the best response was observed with an mRNA booster.

The possibility of combining different vaccines is good news, as it will add flexibility to vaccination strategies in countries where there may be shortages of certain vaccines.

Last updated on November 23, 2022.

Do we need a fourth dose?

For the 2022-2023 winter, European countries started by offering a second booster (or fourth dose) with bivalent vaccines to the most vulnerable groups: immunocompromised persons, nursing home residents and staff, people over 60 years of age, people with health conditions that put them at higher risk of severe disease, and pregnant women. Healthcare personnel is another priority group, since they are more exposed. Once these groups were vaccinated, a booster dose with the bivalent vaccines is being recommended to the rest of the population, to increase protection against the Omicron sublineages that are currently circulating. In any case, it is recommended to wait for 5-6 months after the last infection or booster dose.

Last updated on January 10, 2023.

Are there vaccines adapted to the new variants?

Yes. Both Moderna and Pfizer have developed so-called bivalent vaccines, which combine mRNA for Spike of the original variant and of either Omicron BA.1 (developed first) or Omicron BA4/5 (the most recent).

The four bivalent vaccines have been approved by the EMA for use as a booster (and even for primary vaccination), in people aged over 12 (over 5 for Pfizer). Early results show that these vaccines induce good levels of neutralising antibodies against currently circulating Omicron subvariants, including BQ1.1 and XBB1.5.

Sanofi Pasteur's VidPrevtyn Beta vaccine has also been auhorised by EMA as booster dose in people aged 18 or more. This vaccine contains the Spike protein of the Beta variant of SARS-CoV-2.

Hipra's vaccine, also protein-based,is under review by EMA. Ut is also a bivalent vccine, but in this case it contains the Spike protein from Alpha and Beta variants. It could also be a good choice as a booster, since it induces a robust and sustained neutralizing response against BA.2, BA.4 and BA.5 subvariants, according to data from the company.

Last updated on January 10, 2023.

What will second-generation vaccines look like and when might they be available?

Currently available COVID-19 vaccines have proven to be very safe and effective. However, protection against severe disease appears to decline over time (especially in older populations), they are less effective against new variants (particularly omicron) and they fail to stop viral transmission (especially with Omicron). Therefore, second-generation vaccines that are adapted to the new variants or that are able to protect against a wide range of coronaviruses ( "pancoronavirus" vaccines ) will be needed, as well as vaccines that are administered intranasally to boost mucosal immunity and decrease transmission. It will also be important to have vaccines that are easier to produce locally, more affordable, and more stable at ambient temperatures to facilitate access and distribution in low-income countries.

Some of these second-generation vaccines are already a reality. Pfizer and Moderna have adapted their mRNA vaccines to include the Omicron (BA.1 or BA.4/%) sequence in their formulation. China and India have approved nasal COVID vaccines to be used as booster and there are almost 100 other intranasal vaccines in development. With an adequate support, there are vaccines that could be produced easily and affordably in low and middle-income countries and that have shown to be safe and effective, such as a recombinant protein-based vaccine and a viral vector-based vaccine.

Last updated on September 9, 2022.

Is it possible to develop a vaccine against all variants of SARS-CoV-2?

Coronaviruses represent a real global health threat because of their wide geographic distribution, the number of animal reservoirs, and their ease to infect humans. In the past 20 years, three serious outbreaks due to previously unknown betacoronaviruses have occurred: the SARS outbreak (in 2002-2003), the MERS outbreak (since 2012), and the COVID-19 outbreak (since 2019). Therefore, developing a universal vaccine that protects us not only against all SARS-CoV-2 variants, but also against other coronaviruses that could cause future outbreaks, has become a priority .

Previous studies indicate that it is possible to develop a vaccine that targets, if not all coronaviruses, at least those belonging to the betacoronavirus family (such as SARS, SARS-CoV-2 and MERS). It has been shown, for example, that serum from people who passed COVID-19 is able to neutralize (cross-neutralize) SARS or MERS. This is because all coronaviruses share certain genes (or gene segments) that are quite stable (i.e. do not change much over time). There is still a lot of work ahead to define which antigens to use in the vaccine and how to administer them, and many groups are actively working on this. The CEPI coalition has awarded almost 200 million USD to 11 institutions or companies that are exploring different ways of developing these broadly protective vaccines that could prevent a future coronavirus pandemic. For the moment, a vaccine developed by the Walter Reed Institute and currently in advanced clinical phases has shown promise as a platform for a universal vaccine against beta-coronaviruses.

Last updated on September 9, 2022.

2. Side effects

What are the side effects of these vaccines?

The most frequent side effects are pain at the injection site, fatigue, headache and in some cases fever or chills. There is no reason to worry about these side effects, on the contrary, they mean that your immune response is kicking in.

Occasionally, there can be more serious adverse effects. But a vaccine is approved only if it shows that its benefits largely outweigh the risks. Some of these serious adverse effects are so rare that they are only observed once the vaccine is used at a very large scale.

mRNA vaccines (Pfizer and Moderna) : Some cases of anaphylaxis have been reported, with the mRNA vaccines but they are fortunately very rare and no deaths were reported. Cases of myocarditis (heart inflammation) have also been associated with mRNA-based COVID-19 vaccines, particularly after the second dose. The incidence is higher among young men (around 4 per 100,000 people) than in females (less than 1 per 100,000). Cases tend to follow a benign course and recover after a short hospital stay.

Adenoviral vaccines (Oxford /AstraZeneca and Johnson & Johnson) : Rare cases of thrombosis have been associated with these vaccines. The reported incidence varies between countries but is around 10 per 1 million for the first dose and 2 per 1 million for the second dose, for the Oxford/AstraZeneca vaccine. For the Johnson & Johnson vaccine, it is around 2 per 1 million doses, and seems to be more frequent in younger ages. These cases share unusual clinical characteristics (brain or abdominal blood clots with low platelet counts, together with antibodies that recognise and activate platelets), and can be fatal if they are not treated promptly. However, the risk of falling sick or dying from COVID-19 continues to be much higher than the risk of vaccine-related clots, particularly in people aged above 40 and in regions with high incidence of cases. Some very rare cases of Guillain-Barré syndrome (muscle weakness or paralysis) following vaccination with these adenoviral vaccines have also been reported (less than 1 in 1 million). Most cases make a full recovery.


Last updated on November 23, 2021. 

Are the side effects stronger after the second dose?

For the mRNA-based vaccines, the side effects seem to be stronger after the second dose. For the AstraZeneca vaccine, they seem to be stronger after the first dose. Are they more frequent among elderly people?


Last updated on April 27, 2021.

Are side effects more frequent among elderly people?

No, on the contrary. Elderly people have a less reactive immune system and therefore have less side effects.


Last updated on April 27, 2021.

How long do side effects take to appear? How long do they last?

The side effects normally appear the same day or the day after the shot and can last a couple of days. Regarding the more serious adverse events, anaphylactic reactions occur within the first 15 minutes following vaccination. Myocarditis usually occurs within the first week of vaccination. The rare cases of thrombosis with thrombocytopenia normally occur within the first two to three weeks after vaccination (14 days in average), as well as the rare cases of Guillain-Barré syndrome.


Last updated on November 23, 2021. 

3. About the global vaccination efforts

Why is it important that vaccines reach all countries?

Ensuring equitable access to COVID-19 vaccination in all countries is not only an ethical imperative and an obvious international economic necessity, but also the most effective strategy for controlling the pandemic and dealing with the emergence of new variants of the virus.

Pockets of population susceptible to infection and transmission facilitate the emergence of variants, often with greater infectious capacity, which in a globalised world are almost impossible to contain geographically. There is an urgent need for a massive global distribution of vaccines, which would not only reduce the burden of disease in resource-poor countries, but also minimise the evolution of the virus itself and thus the possibility of uncontrolled epidemic waves such as the one caused by the Omicron variant.

Last updated on January 28, 2022.

How is the COVID-19 vaccination effort going, worldwide? How many vaccine doses have been administered in different countries?

Source: Our World in Data based on public official sources.

What is being done to ensure that all countries have access to vaccines?

"No one is safe unless everyone is safe". This is the motto of the COVAX alliance, an initiative launched at the beginning of the pandemic by various public and private actors to ensure equitable access to COVID-19 vaccines. Its most basic commitment was to ensure that at least 20% of the population in all countries would be immunised by 2021. While it has fallen short of its targets, the partnership has managed to deliver a total of 900 million doses to low- and middle-income countries by 2021. Despite this, the reality is that the vaccines have not reached most of the world. For this reason, complementary initiatives have emerged, such as the African Vaccine Procurement Team for Africa, promoted by the African Union, which aims to vaccinate 60% of the continent's population by 2022 and has already managed to deliver 400 million doses.

However, as long as richer countries continue to dominate the distribution of vaccines, it will be difficult for demand to ease. This tension only undermines the bargaining power of countries with fewer economic resources, which will continue to be at the mercy of bilateral donation agreements that do little to address the problem.

Last updated on January 28, 2022.

4. About child vaccination

Why do children need to be vaccinated against COVID-19 if the disease is usually mild or asymptomatic in this age group?

Although severe disease resulting from SARS-CoV-2 infection is rare in children, this does not mean that it does not exist. Vaccination can prevent acute disease and, importantly, also the chronic consequences of the infection, such as the so-called Long Covid or the multi-inflammatory systemic syndrome (MIS-C), a rare but potentially serious complication that affects this age group. In addition, childhood vaccination contributes significantly to reducing the amount of individuals susceptible to infection, thereby reducing viral transmission.

Last updated on February 1, 2022.

What is the point of vaccinating the paediatric population if current vaccines do not prevent infection by omicron?

Although it is true that current vaccines do not fully protect against infection by the omicron variant, vaccinated people have a lower risk of becoming infected than unvaccinated people. Moreover, vaccination primarily protects against the individual risk of disease, and for this reason alone it is justified. There are many examples of other paediatric diseases where the risk of serious illness is even lower than with COVID-19, and yet we continue to vaccinate children.

Last updated on February 1, 2022.

Are COVID-19 vaccines safe for children?

We can categorically affirm that COVID-19 vaccines are safe and well tolerated in the paediatric age group. No serious adverse effects have been detected in the nearly 9 million US children who have already been vaccinated, as well as in the nearly two million Spanish children who have already received their first dose.

Last updated on February 1, 2022.

What are the side effects of the paediatric COVID-19 vaccines?

Side effects are mild, typically local (at the injection site) and self-limiting. General malaise and low-grade fever may also occur, but in general the vaccines in this age group are very well tolerated.

Last updated on February 1, 2022.

Have serious side effects in children been reported?

Potentially serious side effects that have been observed in other age groups, such as myocarditis, which is more frequent among young adults or older adolescents (especially boys), have a negligible incidence in children under 12 years of age. No other effects of concern have been reported in this age group.

Last updated on February 1, 2022.

Are the COVID-19 vaccines likely to have long-term adverse effects in children?

Due to the global emergency situation, COVID-19 vaccines have been deployed very quickly, without long-term data on their safety. However, this is not an anomalous procedure, or lacking in rigour, as this has also been the case for other vaccines used in paediatrics. In any case, there is no indication that these vaccines have any long-term effects, nor that they may interfere with the physical or mental development of those who receive them.

Last updated on February 1, 2022.

If my child has already had COVID-19, does he/she need to be vaccinated? When?

Yes, vaccination is required also in these cases. If a child has already had COVID-19 and the infection is confirmed by a reliable diagnostic test, only one dose of the vaccine should be given, starting 8 weeks after the infection has passed. The only exception would be children with some form of immunosuppression, who should receive the full course (two doses of primary vaccination, plus the additional dose), even if they have been previously infected.

Last updated on February 1, 2022.

What if my child gets infected after having received one dose?

When the infection occurs after the first dose, the second dose can be given, but it is better to wait 8 weeks after the infection.

Last updated on February 1, 2022.