Reasons for booster vaccinations against COVID-19

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Why is booster vaccination necessary for COVID-19?

Vaccinations against COVID-19 went extremely well in Iceland last spring, faster than in many of our neighbouring countries, and participation here was also higher than in many other places.i At the same time as vaccinations were in full swing, there were reports of COVID-19 virus variants carrying the mutated S-protein, in some cases modified enough to reduce the effect of the vaccine when using vaccines based on that protein as has been the case here.ii The Delta variant, which is now dominant in Iceland and elsewhere, is one such variant.iii Despite this, two vaccinations are quite effective against infection, as an individual living with a COVID-infected individual is about 50% less likely to be infected if they are vaccinated than otherwise.ivVaccinated individuals who get infected despite being vaccinated are less likely to infect others in the same household,v probably because they are less contagious than unvaccinated individuals.iv Other countries that had been successful in vaccinations were heavily affected by the Delta variant this summer, just as Iceland was. The Israelis were the first to recommend a booster dose of a vaccine from Pfizer/BioNTech when they had data showing that individuals vaccinated more than five months ago were more likely to be infected with the Delta variant than those who had completed the vaccination within five This was followed by research from the manufacturervii that confirmed the benefit of a booster dose to reduce the deterioration of the antibody response in fully vaccinated individuals and changes in marketing authorisations, and now the Icelandic patient information leaflet states that there may be cause for a booster vaccination 6 months after primary vaccinationviii. Studies in Israel suggest that the real benefit of a booster dose to reduce serious illness due to COVID-19 6 months after primary vaccination is significant.ix Data from the manufacturerx and independent researchersxi suggest that antibody responses, including so-called neutralising antibodies, which are a better measure of infection control than simple quantification of antibodies, are up to ten times more potent after a booster dose three to six months after primary vaccination than soon after primary vaccination. This means that even against the Delta variant, the societal resistance to widespread infection will be much better when a booster dose is given, as seems to be the case in Israel.ix

The need for a booster dose of a vaccine to enhance or prolong protection is well known for many diseases that are vaccinated against, and it is not always clear when marketing how quickly or frequently a booster is needed. An example is the hepatitis B vaccination, where two doses at a monthly interval do not provide good long-term protection, but a third dose must be given approx. six months to one year after the first.xiii

Good participation in booster vaccinations against COVID-19, comparable to that of the primary vaccination last spring, is one of the main prerequisites for us to be able to control the spread now without significant social restrictions. At present, booster vaccination is only available to certain groups, but it is being examined whether there is a reason to offer it to everyone when a certain time has elapsed since the primary vaccination. Participation has been very uneven and in the capital area; only about 20–50% attend when notified. This participation is unlikely to yield significant societal benefits from the booster vaccination, although the benefits to the individual are clear.

Why are only Pfizer/BioNTech and Moderna vaccines used for the third dose in Iceland?

Currently, the most experience is with the booster dose of the Pfizer/BioNTech vaccine after primary vaccination with the same vaccine, as no other vaccine was used in Israel. Studies of the use of different vaccines in two-dose primary vaccination show that a Pfizer/BioNTech vaccine stimulates a very good response to a single dose of AstraZeneca vaccine.xiv Studies on cross-stimulation of a third dose of Pfizer/BioNTech vaccine and Moderna vaccine are underway,xi but these vaccines have been used alternatively and are widely considered equivalent, including in Iceland, in Canadaxv and in Norway.xvi The Moderna vaccine has now also received a marketing authorisation for a booster dose, but with half the amount of vaccine in the booster dose than is used for primary vaccination.xvii The AstraZeneca vaccine has also been studied with regards to the third dose and has shown a stimulant effect,xviii but is unlikely to be used for this purpose here, as the benefits of using the mRNA vaccines from Pfizer/BioNTech and Moderna appear to be greater.

What is known about the side effects of the third dose?

The side effects of the third dose of Pfizer/BioNTech vaccine appear to be comparable to the side effects after the second dose of the primary vaccine. This is based on the healthcare centre’s experience in Iceland of the third dose, data from the manufacturerxix and experience in Israel.xi Foreign data on the side effects of the Pfizer/BioNTech booster vaccination when the primary vaccine was with another vaccine are scarce, but initial indications are that they are probably no worse than when the primary vaccine was given with the same vaccine.xi In collaboration with the Icelandic Medicines Control Agency, the Chief Epidemiologist will monitor for side effects after booster vaccinations in Iceland


xii (aukaverkanir á glærum 36-41)

Fyrst birt 16.11.2021

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