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  • Increased incidence of menstrual disturbances after coronavirus vaccination in girls aged 12-15

Research findings

Increased incidence of menstrual disturbances after coronavirus vaccination in girls aged 12-15

Menstrual disturbances among girls aged 12–15 are common. However, the incidence is higher among those who have received one vaccine dose with Comirnaty, according to a new study from the NIPH.

Menstrual disturbances among girls aged 12–15 are common. However, the incidence is higher among those who have received one vaccine dose with Comirnaty, according to a new study from the NIPH.

The first reports that several women experienced menstrual disturbances after coronavirus vaccination came early in summer 2021. The NIPH promptly introduced questions about menstrual disturbances into ongoing population studies. The first results indicated that coronavirus vaccination can affect menstruation in women between the ages of 18 and 30.

A new study from the NIPH published in Vaccine also shows an increased incidence in girls between 12-15 years of age.

"4.7 per cent of the participants reported that their last period before vaccination was heavier than they usually experience. After vaccination, 7.3 per cent reported that the first period after the vaccine was heavier than usual, says Dr. Lill Trogstad, project manager.

"The same pattern was seen for longer duration of menstruation, shorter interval between menstrual cycles and for worse period pain than usual," she adds. See table 1 further down in this article.


Here are the results of the study:

  • Around 80 per cent of girls in this age group received one vaccine dose. Almost all the girls in the study had received Comirnaty (BioNTech and Pfizer).

  • The study shows that menstrual disturbances among girls aged 12–15 are common. 22.6 per cent of the girls in the study had experienced disturbances during their last period before

  • The proportion who experienced heavier menstrual bleeding than usual was higher after vaccination (7.3 per cent) than before vaccination (4.7 per cent).

  • Only 1.5 per cent of the vaccinated girls had had a COVID-19 infection during the time when the study was carried out. Among the girls who were not vaccinated, 28 per cent had had a COVID-19 infection.

  • We saw an indication that unvaccinated girls who had COVID-19 infection also had an increased incidence of menstrual disturbances, but the association with infection should be followed up by further studies.

Over 7,000 MoBa mothers participated in the study

Mothers in the Norwegian Mother, Father and Child Study (MoBa) were asked whether their daughters had experienced menstrual disturbances before and/or after the first dose of the coronavirus vaccine. The results are based on questions that 7,565 MoBa mothers answered on behalf of their children between 11-15 October 2021. At that time, adolescents aged 12–15 who had not had COVID-19 infection were recommended to be vaccinated with one dose of Comirnaty (BioNTech and Pfizer).

The main purpose of this study was to find out whether menstrual disturbances occur more frequently among vaccinated than non-vaccinated girls.

This is the first study to look at menstrual disturbances in this age group. The findings should be confirmed in further studies.

Several events were studied

The events studied were:

  • unusually heavy menstrual bleeding
  • unusually prolonged periods
  • unusually short interval between periods
  • unusually long interval between periods
  • unexpected breakthrough bleeding
  • unusually painful periods
  • period-like pain without bleeding

The questions used to clarify this were:

  • Did she experience any of the following during her last period before vaccination?
  • Did she experience any of the following during her first period after vaccination?
Table 1. Reported bleeding patterns among 7,565 menstruating girls aged 12–15 years before and after the first vaccine dose.

Menstrual disturbances

Before 1st vaccine dose (unvaccinated)*

After 1st vaccine dose (vaccinated)

Heavier than usual

4.7 %

7.3 %

Longer duration than usual

3.9 %

5.4 %

Shorter interval between periods



Longer interval between periods



Unexpected breakthrough bleeding



More painful periods than usual



Period-like pain without bleeding



*The uncertainty (95% confidence interval was +/- 1% for all menstrual disturbances)

Higher risk after vaccination

There was no difference before and after vaccination in the incidence of prolonged intervals between menstrual cycles, breakthrough bleeding or period pain/cramps without bleeding after the first dose.

The results did not change when the girls who had previously had COVID-19 infection were excluded from the analyses. The study does not provide information on the duration of the menstrual disturbances.

We found a higher risk of experiencing heavier menstrual bleeding than usual after vaccination with one dose, with a relative risk (RR) of 1.60 (95 per cent confidence interval 1.43-1.80). A higher risk of longer duration of bleeding was also seen, with a relative risk of 1.39 (95 per cent confidence interval 1.22-1.59), see table 2.

Table 2
Menstrual disturbances

Relative risk and 95% confidence interval after the 1st vaccine dose

Heavier than usual

1.60 (1.43 til 1.80)

Longer duration than usual

1.39 (1.22 til 1.59)

Limitations in the study

The answers in this study are based on the mothers' report of menstrual disturbances in their daughters. There has been a lot of media attention around the signals about menstrual disturbances after coronavirus vaccination, and this may also contribute to awareness among more mothers and/or girls reporting menstrual disturbances after vaccination, even though they may have previously experienced the same without vaccination.

Studies initiated at NIPH after reports of menstrual disturbances

The Norwegian Medicines Agency publishes regular updates of side effect reports after coronavirus vaccination:

Early in summer 2021, the first reports came that several women experienced menstrual disturbances which they attributed to vaccination. To investigate any connection between coronavirus vaccination and menstrual disturbances, the NIPH began a large study among more than 60,000 women aged 12–80 in Norway. Using electronic questionnaires, the participants answered questions about menstruation and menstrual disturbances.

Follow-up and data analyses continue both in MoBa and in the other study cohorts. This will be able to provide answers about menstrual disturbances and bleeding in other age groups, and about the duration of the problems. Data from the studies also showed an increased incidence of menstrual disturbances in young women aged 18-30 after vaccination against coronavirus

The contribution from the participants is extremely important and provides unique knowledge about conditions that cannot be obtained from national health registries. We are very grateful for the effort and patience the participants have shown through the pandemic.

International studies on menstrual disturbances after vaccination

Side effect reports on menstrual disturbances have been discussed in the European Medicines Agency's Committee for Pharmacovigilance - PRAC (Pharmacovigilance Risk Assessment Committee). The PRAC has concluded that heavy menstrual bleeding is a possible side effect after COVID-19 vaccination and will continue to monitor this.

Menstrual disturbances after COVID-19 vaccination have been discussed in several comment articles in the British Medical Journal (BMJ) this autumn, also from the NIPH:

In the USA, a large study has also been launched to investigate whether women's menstrual cycles can be affected by coronavirus vaccination. COVID-19 Vaccines and the Menstrual Cycle . A multinational study in which women self-register their periods in an app reported that COVID-19 vaccination was associated with a small disturbances in cycle length, but not the duration of the period.

Menstrual disturbances are rare after other vaccines

In Norway, the Norwegian Medicines Agency has not received many reports of menstrual disturbances linked to the use of other vaccines. There are very few scientific publications on menstrual disturbances after other vaccines.

Menstrual disturbances are very common in girls in the first few years after they start menstruating. We know that menstruation in women can generally be affected by many factors, for example stress, infections, medicines, hormones/contraception, fibroids, endometriosis or other diseases in the uterus and cervix. For girls aged 12-15, many of the factors are irrelevant due to age. However, we know that when many people are vaccinated, it is expected that some will randomly experience menstrual disturbances and unexpected menstrual bleeding in the time related to vaccination.

Vaccination advice for the age group 12-15 years

There is currently no recommendation to vaccinate healthy girls and boys in the age group 12–15 years, but a vaccine is available if the guardian or the child/adolescent wants it. There are separate recommendations for children and young people with serious underlying disease, see Vaccination of children and young people

Transient menstrual disturbances after vaccination with the 1st dose of coronavirus vaccines is not a contraindication to vaccination with a 2nd dose, if this is recommended due to a serious underlying condition or at your own request. If there are persistent menstrual disturbances, the cause should be investigated and further vaccination carried out in consultation with your own doctor.