CHANGES in women's periods following Covid vaccination are "short-lived and small", according to new research.

In the UK, more than 36,000 reports of menstrual changes - such as an unusually heavy or painful period, or unexpected vaginal bleeding following Covid-19 vaccination - have been reported to date via the yellow card surveillance scheme run by the UK Medicine and Healthcare Products Regulatory Agency.

However, this data is self-reported and does not provide a comparison with changes occurring naturally in the unvaccinated population.

Two new studies, from the US and Norway, provide "reassuring" results, according to Dr Victoria Male, an expert in reproductive immunology at Imperial College London.

Writing today in the BMJ, Dr Male said the evidence indicates that changes "do occur" following vaccination, but these are "small compared with natural variation and quickly reverse".

READ MORE: Menstrual changes linked to Covid vaccines 'likely to be short-lived'

In the US study, nearly 4000 Americans - around 2,400 inoculated using either the Pfizer or Moderna Covid vaccines, and nearly 1,600 unvaccinated - logged details of six consecutive menstrual cycles in a tracking app.

After accounting for other factors, the first vaccine dose had no effect on timing of the subsequent period, while the second dose was associated with a delay of 0.45 days.

The group most affected were 358 women who received both doses in the same cycle - something that cannot happen in the UK where doses are administered at eight week intervals, rather than three to four weeks.

For this group, period onset was delayed by around two days on average, and by more than eight days for 11% of participants compared to 4% in the unvaccinated, control group.

No difference was found in relation to the duration of bleeding and menstrual cycles had returned to normal within two months of vaccination.

In Norway, a study involving nearly 5,700 participants found that changes to normal cycle patterns - such as unexpected bleeding or worse-than-usual pain - were reported almost as frequently for the period immediately before vaccination as the ones occurring after a first or second dose.

The rates were 38%, 39% and 41% respectively, with a heavier than normal period being the most common post-vaccination change.

Like the US, Norway uses the Moderna and Pfizer vaccines administered three to four weeks apart.

The UK is due to publish its first data, using the same menstrual tracking app as the US study, which will enable researchers to determine what impact - if any - an eight-week interval between doses has on menstrual cycles.

The MHRA says that current evidence "does not support a link between changes to menstrual periods and Covid vaccination in the UK", although there is evidence that the Covid infection can disrupt menstruation.

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Dr Male said public concern has arisen from "misinformation that Covid-19 vaccines cause female infertility."

She writes: "Although we already have evidence that this is not the case, it comes from the clinical trials, in which pregnancy rates were extremely low because participants were using contraception, and fertility clinics, where users do not necessarily reflect the broader population.

"Studies of pregnancy rates in couples trying to conceive through intercourse are needed, and they should also include analyses of the effects of having Covid-19, because evidence suggests that infection may reduce sperm count and quality."

Dr Male added that the latest research represents a "step in the right direction", but should have been prioritised earlier.

"The fact that it has taken us so long to get here reflects the low priority with which menstrual and reproductive health is often treated in medical research."