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FAQs on COVID-19 Vaccines

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

Last update: June 17, 2021

Photo: Thirdman / Pexels

Thanks to remarkable scientific progress and previous experience with other coronaviruses, several COVID-19 vaccines have already been approved in different countries.

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.

Index

> General questions

> About the side effects

> About the Pfizer / BioNTech vaccine

> About the Moderna vaccine

> About the Oxford / AstraZeneca vaccine

> About the Janssen vaccine (Johnson & Johnson)

> About the Janssen vaccine (Johnson & Johnson)

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.

What are the differences between the vaccines currently tested?

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 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 protect us from disease in case of infection by the virus.

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

These vaccines use a harmless virus to introduce into our cells the genetic 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 consists of a modified chimpanzee adenovirus that carries the DNA coding for the SARS-CoV-2 Spike protein. The adenoviral vector infects our cells and delivers this DNA fragment so that our cells can produce the protein. Our immune system will see it as a foreign protein and will generate virus-specific antibodies and T cells.

The vaccine by Janssen uses a modified human adenovirus called Ad26 that can infect our cells but cannot replicate. This viral vector carries the DNA that codes for the SARS-CoV-2 Spike protein.

Last updated on February 25th 2021.

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 that codes for protein S, which is a SARS-CoV-2 specific protein. That is, these vaccines contain genetic material synthesized in the laboratory that makes our cells react and make copies of said protein S. Afterwards our immune system recognizes protein S as foreign and responds by generating specific antibodies, therefore, when this coronavirus enters our body, we already have immunity and we will not get sick.

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.

Can they give us 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.

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?

Vaccines against COVID-19 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 vaccines approved by the regulatory agencies are absolutely safe, since they have previously completed the three phases established in the clinical trials. This means that when vaccination campaigns begin, tens of thousands of people have already received the vaccine during clinical trials.

The main unknown that remains to be answered as a result of the speed with which clinical trials have been carried out is for how long will these vaccines protect, since at the time they get on the market, their effectiveness will only have been followed for 6-8 months. If their efficacy is observed to decline over time, it will probably be necessary to re-vaccinate.

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 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.

How long will vaccine-induced immunity last?

It is still too early to give a rigorous answer to this question. Efficacy data from phase III clinical trials of each of the vaccines is beginning to be released. Later, it will be necessary to do follow-up to evaluate the effectiveness of the different vaccines over time.

Recording date: 26-27 November, 2020.

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

Although several of the most advanced vaccines have shown an efficacy above 90%, no vaccine is 100% effective. Therefore, we may expect that some people will fall sick with COVID-19 despite being vaccinated and we cannot rule out the possibility that some may even die. But the great majority of vaccinated people will be protected against the disease, or against severe forms of the disease.

Updated on 17 December, 2020.

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. Therefore, it is a question for which there is still no definitive answer.

However, there is preliminary but promising evidence that vaccines will also help reduce viral transmission. First, vaccinated people have been shown to have a lower risk of infection (e.g. a study in the USA with almost 4,000 health workers shows that the risk of infection dropped about 90% after the second dose of mRNA vaccine). Similarly, a study carried out with health personnel in Catalonia and another study in Israel showed a 85-96% reduction in infections among people vaccinated with two doses. In addition, people who become infected after receiving the vaccine seem to have a much lower viral load, even after a single dose. Finally, a recent study in Great Britain shows that people who became infected after receiving the first dose of vaccine transmitted the disease to fewer contacts than unvaccinated people — household transmission  decreased about 50%. Transmission is likely to be further reduced after both vaccine doses. A study in Israel shows that for every 20% of people vaccinated in the community, the number of infections in unvaccinated people is reduced two-fold.

But reducing is not the same as eliminating, so vaccinated people should continue to apply preventive measures (use of masks, social distance) until a sufficient percentage of the population is vaccinated.

Last updated on April 29th 2021.

Publishing date: 4 May 2021.

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

It is now clear that the vast majority of persons who have been infected develop a protective immunity. The duration has not been yet established, but in most cases it is of at least six months and probably more. This is why, in a context of limited availability of vaccines, these people should not be prioritised for vaccination.

It is now clear that the vast majority of persons who have been infected develop a protective immunity. The duration has not been yet established, but in most cases it is of at least six months and probably more. This is why, in a context of limited availability of vaccines, these people should not be prioritised for vaccination.

Last updated on February 25th 2021.

Recording date: 26-27 November 2020.

Is it possible to combine two different vaccines?

This possibility is currently being evaluated and would add flexibility to the vaccination strategies.

The UK launched a clinical trial combining one dose of AstraZeneca with one dose of Pfizer or other vaccines, with different intervals and in different order. Preliminary results suggest that combining vaccines at a 4-week interval increases the frequency of side effects, although they are temporary and mild. Efficacy results have not been published yet.

The ISCIII in Spain announced preliminary results for its CombivacS study, showing that a second dose of Pfizer 8-12 weeks after a first dose of AstraZeneca is safe and induces good levels of anti-Spike antibodies. These results are encouraging, but do not allow to know what the efficacy of this combination will be (i.e. how much will it protect from disease).

Last updated on May 20th 2021.

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.

In this sense, some cases of anaphylaxis have been reported, particularly with he mRNA vaccines (Pfizer or Moderna), but they are fortunately very rare: 66 cases after 18 million doses administered, according to updated data from the US. All (except one) occurred in the first 11 minutes and no deaths were reported.

Additionally, some rare cases of thrombosis were observed after millions of doses of the Oxford /AstraZeneca and Johnson & Johnson vaccines were administered (around 200 cases out of 34 million doses for AstraZeneca, and 15 cases out of 7.5 million doses for Johnson & Johnson). These cases share unusual clinical characteristics (brain or abdominal blood clots with low platelet counts, together with antibodies that recognise and activate platelets), similar to what is observed with heparin-induced thrombocytopenia, suggesting there is a causal association. These cases, however are very rare (around 4 in 1 million) and can be treated if detected promptly. Therefore, 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.

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?

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.

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. The rare cases of anaphylaxis usually occur within the first 15 to 30 minutes after the shot and are treated with the administration of epinephrine.

Questions on side effects last updated on March 2, 2021.

3. Pfizer / BioNTech vaccine

What is the efficacy of the Pfizer / BioNTech vaccine?

According to the clinical trial results that led to its approval, the efficacy of the Pfizer / BioNTech vaccine (in terms of protecting against symptomatic disease) is 95%. These clinical trials were conducted in controlled conditions (which is why we speak of efficacy and not effectiveness) and when the original viral variant was predominant. Results from a study in Israel (with more than 600,000 people vaccinated) confirm that the vaccine effectiveness (i.e. in real life conditions) is very high (94%) in people who have received both doses, even against the B1.1.7 variant (which was first identified in Great Britain).

Regarding protection against severe disease, data from Israel (where, to date, more than 70% of the adult population has been vaccinated) show a reduction of 98.9% in COVID-19 hospitalizations and deaths.

Does the Pfizer / BioNTech vaccine protect against the new viral variants?

Experiments in the laboratory and data from Israel and Great Britain show that the Pfizer vaccine is almost as effective against the “British” B1.1.7 variant as compared to the original variant.

Although it has not yet been tested in South Africa or Brazil, experiments in the laboratory suggest that the neutralization capacity by vaccine-generated antibodies is reduced (up to 7-fold) against the variants that were first identified in these countries (B1.351 and P.1).

The real effectiveness of the vaccine against these variants remains to be determined, but it is hoped that it will remain effective due to the high level of antibodies induced by the vaccine, and, most importantly, that it will still protect against severe disease and death.

To date, no hospitalizations or deaths by COVID-19 have been registered among people who received both doses of the Pfizer / BioNTech vaccine in clinical trials.

How long does it take to be protected by the Pfizer / BioNTech vaccine?

Data from Israel show that, by two to three weeks after the first dose, the number of infections by SARS-CoV-2 is reduced by 75%. However, this does not mean one dose is enough since it is not clear how long the protection would last after one single dose. Maximal protection is observed from 7 days after the second dose.

Questions on the Pfizer / BioNtech vaccine last updated on March 2, 2021.

4. Moderna vaccine

What is the efficacy of the Moderna vaccine?

According to the clinical trial results that led to its approval, the efficacy of the Moderna vaccine (in terms of protecting against symptomatic disease) is 94%. These clinical trials were conducted in controlled conditions (which is why we speak of efficacy and not effectiveness) and when the original viral variant was predominant.

To date, no hospitalizations or deaths by COVID-19 have been registered among people who received both doses of the Moderna vaccine in clinical trials.

Does the Moderna vaccine protect against the new viral variants?

Similar to the Pfizer vaccine, this vaccine seems to remain almost as effective against the “British” B1.1.7 variant as with the original variant. However, it has not yet been tested in South Africa or Brazil, where the other viral variants of concern are predominant.

Experiments in the laboratory suggest that the neutralization capacity by vaccine-generated antibodies is reduced (up to 7-fold) against the variants that were first identified in these countries (B1.351 and P.1).

The real effectiveness of the vaccine against these variants remains to be determined, but it is hoped that it will remain effective due to the high level of antibodies induced by the vaccine, and, most importantly, that it will still protect against severe disease and death.

Questions on the Moderna vaccine last updated on March 2, 2021.

5. Oxford / AstraZeneca vaccine

What is the efficacy of the Oxford / AstraZeneca vaccine?

In the different clinical trials conducted, this vaccine has been shown to protect against symptomatic infections with an efficacy ranging between 60% and 90%. According to an analysis published in The Lancet, this difference may be due to the interval between both doses: a longer interval (12 weeks) protects better (above 80%) than an interval below 6 weeks (under 60%).

To date, no hospitalizations or deaths have been registered among people who received both doses of the Oxford / AstraZeneca vaccine in clinical trials.

An analysis of the phase 3 clinical trial in the US updated on March 25, 2021, (with a total of 32,449 participants and 190 cases) shows the efficacy against symptomatic COVID-19 was 76%, and increased to 85% in adults aged 65 years or older. The efficacy against severe disease and hospitalisation was 100% (there were a total of 8 cases, all in the placebo group). For more information, click here.

Real-world effectiveness: The latest data from Public Health England show that the effectiveness against symptomatic disease is similar for the Pfizer and AstraZeneca vaccines (compared with unvaccinated population): 55-70% with one dose (28 days or more after the first dose) and 85-90% with two doses (14 days or more after the second dose).

Question last updated on May 21, 2021.

Does the Oxford / AstraZeneca vaccine protect against the new viral variants?

According to preliminary results obtained in the UK, this vaccine maintains a high efficacy against the “British” (B1.1.7) variant. In contrast, its capacity to protect against symptomatic infections caused by the variant first identified in South Africa (B1.351) seems to be much lower (around 25%) according to a small clinical trial in the African country. Still, it is hoped that it will protect against hospitalisation and death, since to date no cases of hospitalisation or death by COVID-19 have been reported in people who have received both doses of the vaccine in clinical trials.

How long does it take to be protected by the Oxford / AstraZeneca vaccine?

Preliminary results from Scotland show that the Oxford / AstraZeneca vaccine drastically reduced COVID-19 hospitalizations (by 94%) one month after the first dose, even in persons above 80 years of age. However, this does not mean one dose is enough since it is not clear how long the protection would last after one single dose.

Why did some countries stop vaccinating with Oxford / AstraZeneca? Is there any evidence supporting a link between thrombosis and the vaccine?

After a careful review of the cases and consultation with an independent group of experts, the European Medicines Agency (EMA) has reached the conclusion that the unusual thrombosis cases observed after the administration of the Oxford / AstraZeneca vaccine are to be included as a severe side effect of the vaccine. However, these cases are very rare (estimated at 1 in every 100,000 to 250,000 vaccinated people) and a small percentage of them have been fatal (a total of 222 cases, including 18 deaths, among 34 million people vaccinated, as of April 4, 2021).

Therefore, the EMA continues to consider that the benefits of the vaccine largely outweigh the risks.

The agency has provided advice for identifying and managing these rare cases.

According to a German team , this unusual combination of thrombosis and low platelet counts could be due to the production of antibodies that bind to the platelets and activate them. In this case, the condition can be treated with intravenous immunoglobulin administration.

What age groups should receive the Oxford / AstraZeneca vaccine?

The EMA recommends the vaccine for all adults aged 18 years onwards.

In Europe, some countries decided not to use it in people above 55 years of age because the clinical trials had not recruited enough people in this age group. However, the results obtained in the UK, where a considerable percentage of elderly people have been vaccinated, as well as the clinical trial conducted in the US, confirm that the vaccine is highly effective in people above 55 years of age (around 85% protection against symptoms and up to 100% protection against hospitalization and death). Therefore, there is no reason to set a maximum age limit for this vaccine.

With the confirmation of the association between rare thrombotic events and the vaccine, some countries have decided to recommend against the use of the vaccine in the younger population. Thus, the UK will offer an alternative option for people under 30 years of age. Spain has decided to be even more cautious and limit its use to those above 60 years of age.

Does giving one single dose of the vaccine make sense?

The studies performed in the UK show a good protectio (around 75%) after 21 days of the first shot. However, it is not clear how long this protection would last if only one dose was given (in all the studies a second dose was given maximum 12 weeks after the first). It is also known that the second dose is necessary to increase the titer of virus-specific antibodies. Therefore, one single dose will very likely protect less efficiently and for a shorter period of time.

Is it safe to combine a first dose of AstraZeneca with a second dose of Pfizer?

The UK launched a clinical trial combining one dose of AstraZeneca with one dose of Pfizer or other vaccines, with different intervals and in different order. Preliminary results suggest that combining vaccines at a 4-week interval increases the frequency of side effects, although they are temporary and mild. Efficacy results have not been published yet.

The ISCIII in Spain announced preliminary results for its CombivacS study, showing that a second dose of Pfizer 8-12 weeks after a first dose of AstraZeneca is safe and induces good levels of anti-Spike antibodies. These results are encouraging, but do not allow to know what the efficacy of this combination will be (i.e. how much will it protect from disease).

 

Questions on the Oxford / AstraZeneca vaccine last updated on May 20, 2021. 

6. Janssen vaccine (Johnson & Johnson)

What is the efficacy of the Janssen (Johnson & Johnson) vaccine?

The clinical trial results for the one-dose Janssen vaccine (Johnson & Johnson) indicate that, 28 days after vaccination, protection against symptomatic cases was 72% in the US, 66% in Mexico, and 57% in South Africa. Protection against COVID-19 hospitalization and death was complete (100%) at this time point.

To date, this is the only vaccine whose efficacy has been tested with one single dose, and is currently undergoing an approval process by the European Union.

Does the Janssen (Johnson & Johnson) vaccine protect against the new viral variants?

One of the phase 3 trials for this vaccine took place in South Africa, once the new variant that has spread in the country became dominant. Therefore, we have real vaccine efficacy data against this variant that is believed can partially escape immunity. The efficacy against symptomatic infections caused by this variant (B1.351) was indeed lower than that observed in the US trial (58% versus 72%). However, it is important to underline that protection against COVID-19 hospitalisations and deaths was 100% at 28 days after vaccination, regardless of the variant.

How long does it take to be protected by the Janssen (Johnson & Johnson) vaccine?

Protection against COVID-19 hospitalization and death was complete (100%) from 28 days after vaccination.

Is there a link between the Janssen (Johnson & Johnson) vaccine and the cases of thrombosis?

Some unusual cases of thrombosis have been observed after vaccinating serveral millions of individuals with the Johnson & Johnson vaccine. (15 cases among 7.5 million doses). These cases share some unusual clinical features (brain or abdominal blood clots with low platelet counts, together with antibodies that recognise and activate platelets) similar to what is observed with heparin-induced thrombocytopenia, suggesting there is a causal association. These cases, however are very rare (around 4 in 1 million) and can be treated if detected promptly. Therefore, 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.

Questions on the Janssen vaccine last updated on April 27, 2021. 

7. Novavax Vaccine

What does the Novavax vaccine contain?

The Novavax vaccine (NVX-CoV2373) contains nanoparticles made of the whole Spike protein of SARS-CoV-2, which is produced in insect cells. In order to boost the immune response, these protein nanoparticles are mixed with an adjuvant developed by the company called Matrix-M, a soap-like chemical which is derived from the bark of a tree.

The vaccine can be stored at 2-8 C. It consists of two doses, given 3 weeks apart.

This two-dose vaccine will start to be used once the company has obtained regulatory approval by the FDA (USA) and the EMA (European Union), which is expected to happen within the next two months.

 

Last updated on June 17, 2021.

 

What is the efficacy of the Novavax vaccine?

According to the latest Phase 3 results announced by the company, the vaccine provides 100% protection against moderate and severe disease, and has an overall efficacy of 90%.

These results are from the Prevent trial performed in US and Mexico with almost 30,000 participants. A total of 77 cases were observed: 63 in the placebo group and 14 in the vaccine group. All cases in the vaccine group were mild. Ten moderate cases and four severe cases were observed, all in the placebo group.

The Novavax vaccine showed a 91% efficacy among "high-risk" populations (people over age 65, under age 65 with certain comorbidities or with frequent COVID-19 exposure).

 

Last updated on June 17, 2021.

 

Does the Novavax vaccine protect against the new viral variants?

The Prevent trial was performed at a time when the Alpha (B.1.1.7) variant, first identified in the U.K., became the predominant strain in the U.S, showing it is highly effective against this variant. Efficacy against circulating variants of concern or of interest in the trial was of 93%.

Another trial performed in the UK demonstrated an efficacy of 96.4% against the original virus strain, 86.3% against the B.1.1.7 (Alpha) variant, and 89.7% overall.

Regarding the Beta variant (B1.351) first identified in South Africa, the vaccine has a lower efficacy but seems to protect against severe disease and death. The Phase 2b trial performed in this country showed a vaccine efficacy against Beta of 51% among HIV-negative individuals.

There is still no information on the efficacy of this vaccine against the Delta variant (first identified in India), but data from the UK shows that two doses of other COVID vaccines (AstraZeneca and Pfizer) remain highly effective against hospitalization from this variant.

 

Last updated on June 17, 2021.