Information and context on vaccines. Article written by the science journalists of the SINC agency with the analysis and review of expert sources.

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Most covid-19 vaccines approved inEurope are given in two doses. What happens between the first and second jab?

Hours after receiving the first dose, some of the most common symptoms that may occur, according to the Pfizer– BioNTech, Moderna and Oxford– AstraZeneca vaccine fact sheets, are:

• Pain and/or swelling inthe arm of the injection.
• Headache
• Muscle pain and stiffness
• Joint pain
• Tenths of a fever
• Chills
• Nausea
• Vomiting
• Tiredness or general malaise

“All vaccines and medicines can produce side effects,” César Hernández, head of the department of medicines for human use at the Spanish Agency for Medicines and Health Products (AEMPS), reminds us.

Although the covid-19 vaccines may cause some discomfort, most of the symptoms are mild, short-term and do not generate any type of sequelae. The vaccines are safe, as proven in clinical trials.

The reason for the transient adverse effects caused by vaccination is due to the response of the body's immune system, which begins to generate specific defences against the more severe forms of the disease. After one or two days, these symptoms should disappear.

On the other hand, “serious allergic reactions are very rare,” says Carmen de Mendoza, professor and researcher in the area of Health Sciences at the UNIR and researcher at the Puerta del Hierro University Hospital in Majadahonda, Madrid. “Exceptionally, there may be anaphylaxis reactions after vaccination, a more severe form of allergy or hypersensitivity,” she adds.

If a person in Spain develops an adverse effect like this after vaccination, they should contact their reference health centre or report it directly to the Spanish Pharmacovigilance System for Medicinal Products for Human Use (SEFV-H), which extends the surveillance of vaccines beyond clinical trials.

More discomfort after the second dose

Side effects tend to be more frequent after the second dose, especially localised pain at the injection site, and their frequency decreases with age, as reported in the Vaccine Data Sheets and the Spanish Vaccination Strategy.

For example, in the case of the Pfizer/BioNTech vaccine, the most common adverse effects after the second dose were pain at the injection site (>80%), fatigue or feeling tired (>60%), headache (>50%), myalgia and chills (>30%) and fever and swelling at the injection site (>10%). All of these were mild and disappeared within a few days after vaccination.

After the second dose is administered, the body has already been in contact with the virus antigen contained in the vaccine and may therefore react somewhat more strongly than the first time.

“The response the body generates is a kind of memory of the first one. That's why greater immunity is also achieved,” says Hernández on the possible side effects.

What happens if I get infected between doses?

A person can become infected with the new coronavirus after having received the first dose of the vaccine, as with a single jab the immune response that is generated only provides partial protection.

“If you become infected a few days after receiving the first dose, the immune system will develop a response, even if it is not entirely complete,” says Hernández. “The degree of protection from the first dose varies from person to person,” adds De Mendoza.

In Sapin, in those positive cases of covid-19 that are confirmed after the first dose:

• If it is a person over 65 years of age: wait until he or she recovers and finishes the isolation period and the second dose will be administered, provided that at least the period established has passed between doses.
• If the person is 65 years of age or younger: wait 6 months to complete the vaccination schedule with the administration of the second dose.

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This article is also available in Spanish.