FAQs about the COVID bivalent vaccine

Reading time: 5 min
What is the difference between the "updated bivalent" vaccine compared to the one we have used up to now?

It is a vaccine specifically indicated as a reinforcement dose (“booster”) for people already vaccinated (minimum 2 doses) against the SARS-CoV-2 coronavirus, to reinforce immunity and maintain the prevention of severe forms of COVID-19 in people older than 12 years. 

The vaccine does not contain the whole virus (SARS-CoV-2 coronavirus) and so cannot cause COVID-19

The vaccine includes mRNA adapted to both the original Coronavirus variant (called "Wuhan") and the Omicron variant, specifically the BA.4 and BA.5 types, predominant in recent months.  

This booster dose is expected to provide broader protection against the different virus variants. 

How does the mRNA vaccine work?

The vaccine contains a compound called messenger ribonucleic acid (mRNA) from both the Wuhan and Omicron variants. The mRNA molecule contains the instructions to produce the spike (or S) protein in the virus, which is what the latter needs to enter human cells. 

After receiving the vaccine, some cells read the instructions on the mRNA and temporarily produce the spike protein. The immune system recognises this protein as foreign, then activates T cells (lymphocytes) and produces antibodies to attack it.  

When people come into contact with SARS-CoV-2, their immune system has already been “trained” by the vaccine to quickly recognise the virus and start the defensive response against coronavirus.  

What happens to the mRNA afterwards?

The vaccine mRNA does not remain in the body, but is destroyed several hours after vaccination. 

In no case can the substances injected in the vaccine come into contact with the nucleus of human cells, so there is no risk of any type of genetic alteration. 

How effective is this vaccine?

Both the original and the bivalent vaccine have been shown to be very effective in preventing severe forms of COVID-19, reducing the number of cases of hospitalised patients (both emergency and non-emergency) and significantly reducing mortality from COVID-19. 

Laboratory tests indicate a significant increase in antibodies directed against both the original variant (Wuhan) and against Omicron (BA.1, BA.4 and BA.5) after receiving the updated bivalent vaccine

Can people who have already had COVID-19 be vaccinated with this booster dose?

Yes. Providing at least 5 months have elapsed since their infection. 

How long it takes from the last dose to be able to get the booster?

5 months must have passed since the last vaccine dose was received, whether this was the second, third or fourth dose. 

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

Current vaccines do not prevent contagion or infection as effectively as they did in the beginning. This change is because the currently predominant Omicron variant (especially the BA.4 and BA.5 subvariants) is more transmissible and escapes the action of the immune system better than previous variants. Fortunately, the infection it causes is milder. 

Preliminary data indicate that the booster dose may reduce disease transmission, at least for 3-6 months after administration. 

It is important to try to better adapt our immune response through this vaccine update.  

Will immunosuppressed people be given this vaccine?

Yes. People with immunosuppression constitute a group with a clear indication to receive this booster dose. It is also possible that some immunosuppressed people will respond worse to the vaccine.  

From a safety point of view, there is no additional problem. 

Can pregnant or breastfeeding women be given this vaccine?

Yes. Data gathered over the last 3 years indicate that COVID-19 can be serious for pregnant women and that mRNA vaccines are safe and effective in preventing them catching it. 

There are also sufficient data to demonstrate vaccination and breastfeeding are not contraindicated.  

In addition, antibodies made by the mother can pass to the baby, passively protecting it. 

What are the possible side effects of this vaccine?

The vaccine has the same side effects as the original Pfizer vaccine. They are mostly mild and transient. 

These effects may appear during the first week after vaccination. They are not usual after the first two days. Side effects of these vaccines are very well known, and are some of the most studied in the history of medicine. 

The most frequent mild or moderate effects observed are: 

  • In more than 1 out of every 10 people vaccinated:

    • Fever  

    • Headache 

    • Muscle and/or joint pain. 

    • Pain and swelling at the injection site 

    • General discomfort 

  • In fewer than 1 out of every 10 people vaccinated

    • Chills 

    • Redness 

    • Nausea 

    • Fatigue. 

    • Joint pain 

  • In fewer than 1 out of every 100 people vaccinated: 

    • Local adenopathy 

Painkillers (e.g. paracetamol) or NSAIDs (e.g. ibuprofen) can be taken to mitigate these effects, if necessary 

What is known about cases of post-vaccination myocarditis or pericarditis?

The estimated incidence of myocarditis (inflammation of the heart muscle) or pericarditis (inflammation of the membrane surrounding the heart) in the 7 days after vaccination is 1.3-2.2 cases per 100,000 people per day.  

These side effects are more common in young men (18-25 years of age). 

They are usually clinically mild cases with complete recovery. 

What should I do if I have had an allergic reaction to previous doses of the mRNA vaccine?

Allergy to the vaccine is very rare: 1-2 cases per 100,000 doses administered. 

The vaccine is contraindicated in people who have had a clinically significant allergic reaction to a previous dose of the COVID-19 vaccine or to any of its components. Serious reactions can be: anaphylaxis, angioedema, generalised urticaria, asthma attacks and rhinitis.  

The components that could be responsible for these reactions in people with a history of severe allergy to them are polyethylene glycol (PEG) and polysorbate, which are not included in the vaccine but is closely related to PEG. 

Allergy to tromethamol (an excipient in the updated vaccine) is very rare. This excipient has not been associated with allergic reactions after vaccination. 

Vaccination is not contraindicated in people with allergies to food (including eggs), medications, animals, insects, environmental allergens or latex. 

As with all vaccines, it should always be administered under supervision and with all medical resources at hand to deal with a possible severe allergic reaction. 

Can I have this COVID-19 vaccine simultaneously with the seasonal flu vaccine?

Yes. There is no contraindication for the joint administration of these vaccines. They can be given at the same time. 

If not administered simultaneously, they can also be administered separately, without the need for an interval between them. 

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.


Receive the latest updates related to this content.

Thank you for subscribing!

If this is the first time you subscribe you will receive a confirmation email, check your inbox

An error occurred and we were unable to send your data, please try again later.