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  • Low probability of long-term side effects after coronavirus vaccination


Low probability of long-term side effects after coronavirus vaccination

Over a year has passed since the first coronavirus vaccines became available. More than 11 million doses have been administered in Norway, and over 10 billion doses globally. This provides good insight and knowledge about side effects after coronavirus vaccination.


Over a year has passed since the first coronavirus vaccines became available. More than 11 million doses have been administered in Norway, and over 10 billion doses globally. This provides good insight and knowledge about side effects after coronavirus vaccination.

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The contents of this news article have been subsequently updated and published as part of this article:


Those who are sceptical to vaccines often refer to long-term side effects, indicating that this is something that will appear sometime in the future. But what exactly is a long-term side effect, and how common is it?

Uncommon with side effects later than two weeks after vaccination

For coronavirus vaccines and other vaccines, side effects usually occur within the first few days and the first two weeks. Side effects occurring more than 6 weeks after vaccination are referred to as delayed or long-term side effects. However, it is very unusual for reactions to occur so long after vaccines are given.

This knowledge comes from wide experience from the use of many different vaccines over several decades. Every year since 1952, hundreds of thousands of vaccine doses have been administered through the Norwegian childhood immunisation programme. In addition, a large proportion of the population receives an annual influenza vaccine, and adults are recommended to have regular booster doses of several vaccines, e.g. travel vaccines.

Long-term side effects after vaccination in Norway

Of all the vaccines and vaccine types given in Norway, one vaccine was registered with a serious side effect more than 6 weeks after vaccination. Pandemrix was offered during the swine influenza pandemic in 2009/2010 where some children and adolescents developed narcolepsy after vaccination. 700,000 people under the age of 30 were vaccinated with Pandemrix and 56 were diagnosed with narcolepsy. 60,000 in the same age group were registered as having influenza disease and 16 of the unvaccinated group also developed narcolepsy. More details about this:

How are side effects monitored?

It is difficult to give an exact timeline for how long to follow up an event after vaccination as a possible side effect.

Study participants in clinical studies are monitored for a minimum of one year

When new vaccines are developed, large clinical trials are conducted with many thousands of people, with strict international requirements. Clinical studies give us good knowledge about the most common side effects.

Vaccine manufacturers are also required to follow up the study participants for a minimum of one year after vaccination to give the global community more knowledge about the effect of the vaccine and to detect any long-term side effects.

The Norwegian Medicines Agency has written about follow-up of the coronavirus vaccines' effect, safety and quality after approval

Suspected new side effects are called signals

Suspicion can come from different directions - from those who have received the vaccine, from researchers, or from healthcare professionals. This is picked up either by researchers publishing peer-reviewed medical journals or by people and healthcare professionals reporting suspected side effects, known as spontaneous reporting.

The Norwegian Medicines Agency monitors side effect signals in Norway, and the Norwegian Institute of Public Health assists in the work.

“Extensive international collaboration identifies potential new side effects and follows up any signals received. The signals must be investigated before they can be assessed for a possible causal association with vaccination or not, and this must be both thorough and timely. As a rule, little will be known about what triggers a possible new side effect, when and how often they occur, and whether some groups are more at risk of side effects than others. This is something that the most important studies are trying to find out,” says Gro Evensen, senior adviser at the Norwegian Institute of Public Health

It is very important to be able to compare the incidence of a possible side effect among those who have and who have not received vaccines. However, since many people in Norway have now received a coronavirus vaccine, it is not so easy to compare vaccinated against unvaccinated. Then good health registry data and studies are essential to be able to compare the common incidence of various diseases in the population before and after coronavirus vaccination began.

Good health registries and collaboration allow us to react quickly

Access to reliable health data in the NIPH's emergency preparedness registry Beredt C19 has given us new opportunities to study signals about side effects, and monitor changes in the incidence of various diseases in the population. If many cases of a particular condition are reported after vaccination, it will be possible to quickly investigate using health registry data whether the number of cases of the condition is higher among those who have received the vaccine than usual.

“Compilation of data from national health registries in several Nordic countries has been important in the investigation of rare suspected side effects after coronavirus vaccination. We collaborate on investigations of signals to get a larger, common data source. In this way, we increase the possibility of detecting any changes in the incidence of rare side effects,” Evensen points out.

Stopped vaccination with AstraZeneca

An example of this working well was when vaccination with Vaxzevria (AstraZeneca) was paused in March 2021. At that time, health registry data from Norway and Denmark were compiled, showing an increased risk of a serious and unusual blood clot condition in the brains of vaccinated people compared to those who were not vaccinated. Vaxzevria was removed from the Norwegian coronavirus immunisation programme in May 2021 as a result of these side effects.

People under the age of 30 are now recommended Comirnaty

Another example is the increased risk of heart inflammation after COVID-19 disease. This was also reported among people who had been vaccinated with mRNA vaccines, so the signal was investigated further. The first signals that the mRNA vaccines could cause inflammatory conditions in the heart (myocarditis and pericarditis) came in April 2021. After reviewing the adverse reaction data, myocarditis and pericarditis were added as rare side effects after the mRNA vaccines Comirnaty (BioNTech / Pfizer) and Spikevax (Moderna), and the vaccination recommendations were changed.

“We compiled health registry data from Norway, Sweden and Denmark. They showed a higher incidence of these relatively rare inflammatory conditions among vaccinated than among unvaccinated people. The risk is highest after the second dose for those under 30, especially men, and higher after vaccination with Spikevax than after Comirnaty. As a precautionary principle, we recommend that both women and men under the age of 30 should choose Comirnaty over Spikevax,” says Gro Evensen.

Large-scale population studies provide important insights

In Norway, signals of milder side effects that do not necessarily lead to people contacting the health service are also monitored through NIPH's large ongoing population studies (cohort studies).

When the signal about menstrual disorders appeared in the spontaneous reporting system of the Norwegian Medicines Agency, questions about menstruation were quickly included in these studies for female participants.

The first results from the studies show that coronavirus vaccination can affect menstruation in young women 18-30 years. Based on this, the NIPH issued advice to women who have experienced this:

Number of adverse reaction reports will continue to increase

With such a large proportion of vaccinated people in the population, reports of symptoms or conditions with a suspected association with the vaccine will be received for a long time to come. This is a natural consequence of the notification system.

Any suspected link between an incident and vaccination should be reported in order to identify possible side effects that can then be investigated further. Reports of suspected side effects provide useful knowledge about the vaccines and are important for providing sound vaccination advice. The aim of a vaccine is always to give the best possible effect with the fewest possible side effects.

“For the vaccines we use in Norway, we consider the benefit of protection against a severe COVID-19 disease course to be much greater than the risk of serious vaccine side effects. It would be very unusual and unexpected if new side effects appear that have not yet been seen after 11 million doses have been given in Norway,” says Evensen.

The health authorities continue to monitor the situation closely both in Norway and the rest of the world. Any new signals or suspected side effects that appear that may be related to the vaccines will be thoroughly investigated.