Preguntas sobre la vacuna Janssen (Johnson&Johnson)

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What is its goal?

It is a vaccine for preventing coronavirus disease (COVID-19) in people over 18 years of age with only one injection. The vaccine does not contain SARS-CoV-2 and cannot cause COVID-19.

How does the Janssen vaccine work?

The vaccine is made by using another virus (an adenovirus) that has been modified to carry the gene that gives instructions for producing the S (spike) protein of the virus (the one the virus needs to be able to enter human cells). This adenovirus does not reproduce, nor can it cause any disease.

When we receive the vaccine, some of our cells read the instructions of this gene and 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 our defence response against the coronavirus.

The vaccine requires just a single dose.

How effective is this vaccine?

A clinical trial was carried out with more than 44,000 participants in the United States, South Africa and Latin America. Half of them received the vaccine (vaccinated group) and half were given a placebo (control group).

Two weeks after the injection, the clinical trial showed a 67% reduction (eficacy) in the number of symptomatic cases of COVID-19 in the vaccinated group (116 cases out of 19,630 vaccinated) when compared to the control group (348 cases out of 19,691 unvaccinated).

When do you become immune?

You are immune just 14 days after the single dose.

Can people who have already had COVID-19 be vaccinated with the Janssen vaccine?

In the clinical trial, 2,151 participants who had previously had COVID-19 received the vaccine. 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.

For these reasons, healthcare professionals who have survived COVID can be vaccinated.

The current guidelines recommend waiting at least 6 months from the disease recovery date before vaccinating these people.

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

We still don’t 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 don’t 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.

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 18 be given this vaccine?

The Janssen vaccine is not recommended for children under 18 years of age because there is currently no information about the effects on this age group. The EMA has reached an agreement with Janssen 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.

Why can't pregnant women be given this vaccine?

Studies in animals do not show any adverse effects in pregnancy. However, the vaccines used mostly in pregnancy are those containing messenger ribonucleic acid (mRNA), which are those from Pfizer and Moderna; so these are more recommended during this period.

In the United States, over 100,000 pregnant women have already been vaccinated with mRNA vaccines, and no safety concerns have been identified in either the mother or baby.

Based on the information available to date and accumulated experience, the vaccination of pregnant women is safe. This is especially recommended, given that SARS-CoV-2 infection during pregnancy increases the risk of complications and of developing severe COVID-19 during pregnancy. 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.  Another benefit of vaccinating pregnant and breastfeeding mothers is that the antibodies produced are transferred to the foetus through the breast milk.

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 most frequent mild or moderate effects observed in clinical trials were (source EMA):e: EMA):

In more than 1 out of 10 people vaccinated:

Pain and swelling at the injection site





In less than 1 out of 10 people vaccinated:





In less than 1 out of 100 people vaccinated:



Sore throat

Skin rash

Limb pain


General weakness

In less than 1 out of 1000 people vaccinated:

Skin rash with pruritus


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

What problems could be caused by a serious allergy?

The Janssen 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.

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

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