FAQs about the Pfizer-BioNTech vaccine

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What does the trade name of the Pfizer-BioNTech vaccine mean? (Comirnaty)

The idea is to include the concepts of COVID-19, immunity and mRNA, and to evoke the concept of "community" (COVID-19, RNA, community, immunity).

What is its goal?

It is a vaccine for preventing coronavirus disease (COVID-19) in people over 16 years of age. The vaccine does not contain SARS-CoV-2 and cannot cause COVID-19.

How does the Comirnaty vaccine work?

The vaccine contains a molecule called mRNA (messenger RNA) that includes the instructions for producing the S (spike) protein of the virus (the one the virus needs to be able to enter human cells). When we receive the vaccine, some of our cells read the mRNA instructions and temporarily produce the S-protein. Our immune system recognises this protein as being foreign, so it produces antibodies and activates T-cells (lymphocytes) to attack it. If we later come into contact with the SARS-CoV-2 virus, our immune system will quickly recognise it and initiate a defence response against the coronavirus.

What happens to the mRNA afterwards?

The mRNA in the vaccine does not stay in our bodies. It is destroyed just a few hours after the vaccination. It never comes into contact with the cell nucleus (the whole process takes place in the cytoplasm) and there is therefore no risk of any genetic modification.

How effective is this vaccine?

A clinical trial has been conducted on over 44,000 participants. Half of them received the vaccine (vaccinated group) and half were given a placebo (control group).

The vaccine's effectiveness was determined in more than 36,000 participants aged over 16 who showed no signs of previous infection.

The study shows a 95% reduction in the number of symptomatic cases of COVID-19 in the vaccinated group (8 cases out of 18,198 vaccinated) when compared to the control group (162 cases out of 18,325 unvaccinated).

The vaccine was also 95% effective among participants with risk factors for severe disease caused by COVID-19 (asthma, COPD, diabetes, hypertension, and obesity).

When do you become immune?

Seven days after the second dose, although the development of immunity begins on about the twelfth day after the first dose.

Can people who have already had COVID-19 be vaccinated with Comirnaty?

As part of the clinical trial, 545 individuals who had already had COVID-19 received the vaccine. These people were excluded from the effectiveness analysis, however, no particular or special side effects were recorded among these participants.

There is not enough data to conclude how the vaccine works in people who have already had COVID-19.

The current guidelines recommend waiting at least three months from the disease recovery date before vaccinating these people.

Can the vaccine reduce person-to-person transmission of the virus?

We still do not know. It is not known what percentage of people who have been vaccinated may still carry the coronavirus and transmit the infection.

How long does the protection last?

We still do not know. All the trial participants will continue to be monitored (for up to 2 years) to provide further information on how long the protection lasts. We know that the effectiveness of the vaccine begins after the first dose (about 50%) and peaks (about 95%) one week after the second dose (i.e., 1 month after the first dose, if both doses have been received). Experimental data indicates that immunity to COVID-19 is likely to last for months or years, but not for life.

Why can't children under 16 be immunised with this vaccine?

The Comirnaty vaccine is not recommended for children under 16 years of age because there is currently no information about the effects on this age group. The European Medicines Agency (EMA) has reached an agreement with Pfizer-BioNTech to conduct a trial on this population group at a later stage.

Will immunosuppressed people be given this vaccine?

The data is still limited. It is possible that immunosuppressed people will not respond as well to the vaccine, but from a safety perspective there are no additional problems. Immunosuppressed people can be vaccinated, as they have a high risk of developing severe forms of COVID-19.

Pregnant women can be vaccinated with this vaccine?

Based on the information available to date and accumulated experience, the vaccination of pregnant women is safe. It is also recommended, as SARS-CoV-2 infection during pregnancy increases the risk of complications and the development of severe COVID. It can be administered at any time during pregnancy, and women planning a pregnancy can receive any of the COVID vaccines without having to wait between vaccination and conception.

Breastfeeding women can receive the vaccine without having to stop breastfeeding.  Moreover, an added benefit of vaccinating pregnant and breastfeeding mothers is that the antibodies generated are transferred to the foetus through the breast milk.

According to the Spanish Ministry of Health, the Pfizer/BioNTech and Moderna vaccines are recommended during pregnancy and breastfeeding, but other vaccines may be used in other countries.

What side effects may occur with this vaccine?

Most people who are vaccinated will not experience significant side effects. These may appear during the first week after vaccination. They are not usual after the first two days. The frequent mild or moderate effects observed in clinical trials were (source EMA):

In more than 1 out of 10 people vaccinated:

  • Pain and swelling at the injection site.
  • Tiredness.
  • Headache.
  • Muscle and/or joint pain.
  • Fever.

In less than 1 out of 10 people vaccinated:

  • Redness.
  • Nausea.

In less than 1 out of 100 people vaccinated:

  • Itching at the injection site.
  • Adenopathy.

Painkillers, like paracetamol, can be taken, if necessary, to mitigate these effects.

What problems could be caused by a serious allergy?

The Cominarty vaccine is contraindicated in people who have had an anaphylactic-type hypersensitivity reaction to a previous dose of COVID-19 vaccine or one of its components. The component that currently appears to be responsible for this reaction in people with a history of severe allergy is polyethylene glycol (PEG).

In people with a history of allergic reactions (anaphylactic reactions) to food or drugs (in cases of medicines where PEG or polysorbate has not been the cause), it is not contraindicated.

As with all vaccines, it should always be administered under supervision and with all the medical equipment on hand to deal with a possible immediate serious allergic reaction.

Substantiated information by:

Antoni Trilla
Eduard Vieta Pascual
Gema Maria Lledó Ibáñez
Jacobo Sellarés Torres
Josep M. Miró Meda
Josep Maria Peri
Maica Rubinat
Mariona Violan

Published: 12 March 2020
Updated: 12 March 2020

The donations that can be done through this webpage are exclusively for the benefit of Hospital Clínic of Barcelona through Fundació Clínic per a la Recerca Biomèdica and not for BBVA Foundation, entity that collaborates with the project of PortalClínic.


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