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How many days should elapse between the first and second dose of the vaccine

This article was published on
July 30, 2021

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The available data for covid-19 vaccines establish a minimum period of time between doses. However, some countries have decided to delay the second jab by a few days to make more doses available and protect more of the population.

The available data for covid-19 vaccines establish a minimum period of time between doses. However, some countries have decided to delay the second jab by a few days to make more doses available and protect more of the population.

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

Most of thecovid-19 vaccines approved so far in Europe are administered in two doses (the only exception is Janssen's). Clinical trials, which have demonstrated their efficacy and safety, indicate the period of time that must elapse between the first and second jab in order for a person to develop defences that protect against the most severe forms of the disease.

Based on the results of clinical trials, the number of days between doses varies depending on the vaccine:

  • Pfizer/BioNTech: 21 days
  • Moderna: 28 days
  • Oxford/AstraZeneca: 28 to 84 days

“The first dose elicits a primary immune system reaction against infection. The second dose elicits a much stronger and faster response,” Vicente Larraga, research professor at the Margarita Salas Biological Research Centre of the Spanish National Research Council (CIB-CSIC),explains about these and other double-dose vaccines.

Despite the proven efficacy in these cases, some countries have opted to increase the time interval between doses by a few days more. One of the main reasons for delaying the second injection is to have more initial doses available to vaccinate more people in the first rounds.

“This variation is logical, but because we have been in a hurry, other vaccination patterns haven’t been tried,” Larraga says. Although some vaccines are already licensed and being administered to the population, the fourth and final phase of the clinical trials is still underway to give more depth to our understanding of them. One of these aspects is whether the interval between the two doses can be longer to increase the vaccination capacity of the population.

In the case of Oxford/AstraZeneca, the difficulty of producing vaccines to administer the second dose brought new results. The first dose of the vaccine maintains its immunogenicity for at least 90 days, which would allow the immunised population to be extended before the second dose is needed. Moreover, its efficacy appears to be even greater if one waits for three months, according to a study published in The Lancet.

“It's somewhat strange. I honestly don't have an explanation,” admits Marcos LópezHoyos, president of the Spanish Society for Immunology. “In any case, it seems that immunity does not wane for at least three months, which is positive. And, of course, this in no way means that the second dose should not be given,” adds the head of the Immunology Department at the Marqués de Valdecilla Hospital(Santander).

Based on these observations, the recommendation of the World Health Organisation (WHO) on the use of this vaccine is an interval of between four and twelve weeks between the first and second dose, in line with the stance of the European Medicines Agency (EMA).

The pioneerin this change of strategy was the UK, which decided to separate the two doses of the Oxford/AstraZeneca vaccine by up to three months, based on preliminary results. In Spain, the Public Health Commission of the Ministry of Health has established an interval of ten to twelve weeks between the two doses of the Oxford/AstraZeneca vaccine, preferably 12 weeks.

Depending on the type of vaccine

The Pfizer and Moderna vaccines are vaccines that use messenger RNA to produce the immune response. In contrast, the Oxford/AstraZeneca vaccine is based on a more conventional technology, using a viral vector, in this case chimpanzee adenovirus, to present the new coronavirus to the immune system.

Like the Oxford/AstraZeneca vaccine, there are already other vaccines of this type for other diseases that allow much more to be known, such as the influenza vaccine or the malaria vaccine, which is being piloted in three African countries. In this case, separating the two doses may even lead to more protection.

Regarding the Pfizer and Moderna vaccines, the World Health Organisation allows the second dose to be delayed by up to 42 days (six weeks) in the event of this being necessary; for example, if there are production problems. The European Medicines Agency points in the same direction, although there is a lack of clinical data to administer the second dose beyond what is established in the clinical trials.

For its part, the US Food and Drug Administration (FDA) distances itself from the previous agencies: “The available data continue to support the use of two specific doses of each licensed vaccine at regular intervals,” which are 21 days between the first and second doses for Pfizer, and 28 days for Moderna.

This article is also available in Spanish.

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