Menstrual cycle timing may affect vaccine side effects, according to a study involving 1,474 users of period-tracking app, Clue.

People vaccinated before ovulation were more likely to report side effects than those vaccinated after ovulation.

They also appeared to go longer before a subsequent Covid-19 infection, although the researchers said this finding was exploratory.

Amanda Shea, fractional chief science officer at Clue, said: “We know that immune function fluctuates across the menstrual cycle, yet it’s still rarely considered in mainstream health research.

“To truly understand women’s health, we need to stop treating the menstrual cycle as background noise and start recognising it as a fundamental part of human biology.

“Incorporating cycle data into research is essential to building the evidence needed for more personalised and effective care.”

In the study, participants vaccinated during the follicular phase had 35 per cent higher odds of reporting any vaccine side effect than those vaccinated during the luteal phase.

The follicular phase occurs before ovulation, when oestrogen is dominant. The luteal phase follows ovulation and is dominated by progesterone.

Sensitivity analyses produced similar findings after people vaccinated around menstruation were excluded, suggesting the association was not simply explained by menstrual or premenstrual symptoms.

A sensitivity analysis checks whether findings remain consistent when researchers change parts of their analysis.

The researchers found no evidence that vaccination during the follicular phase was linked to more severe side effects or a greater number of side effects.

Those vaccinated during the follicular phase went a median of 35 days longer before reporting a subsequent Covid-19 infection.

The median was 200 days among those vaccinated during the follicular phase, compared with 164 days for those vaccinated during the luteal phase.

However, only 82 infections were recorded across the sample.

The researchers said the result was exploratory and intended to generate a hypothesis rather than provide evidence that vaccination timing improves protection.

The study was led by Poppy Cooper at the London School of Hygiene & Tropical Medicine and Alexandra Alvergne at the University of Montpellier.

They worked with researchers from Oregon Health & Science University and cycle-tracking company Clue.

The findings add to growing evidence that reproductive hormones may influence immune function.

Oestrogen is generally associated with heightened immune activity, while progesterone tends to have a dampening effect.

Previous studies have found that women often develop stronger antibody responses than men after vaccines for influenza, measles, mumps and rubella, and hepatitis. They also often report more side effects.

Antibodies are proteins made by the immune system that identify and help fight infections.

Clue said research has been central to its work since the company was founded.

Its users have tracked more than 250m cycles and contributed more than 30bn data points with their consent, creating what the company describes as one of the world’s largest long-term menstrual health datasets.

Clue works with researchers at institutions including MIT, Oxford, Columbia and the University of California, Berkeley.

The company said the study was possible because users chose to contribute their tracked cycle data to research.

Rhiannon White, chief executive of Clue, said: “Every time the Clue community tracks their cycle, they’re helping answer a question about their own body that medicine has too often ignored.

“This study is a small example of something bigger: when women are given the tools to understand themselves, and the research catches up to meet them, real change becomes possible.”

Researchers surveyed Clue users about when they received their first Covid-19 vaccine dose, any side effects they experienced and subsequent infections.

The responses were matched with cycle information that each participant had previously recorded in the app, rather than relying on recalled estimates.

Of the 1,474 users included, 760 were vaccinated during the follicular phase and 714 during the luteal phase.

The researchers stressed that people should not use the findings to schedule vaccinations around their menstrual cycle.

They said receiving a vaccine when it is available remains far more important than the phase of the cycle in which it is given.

As the first study to investigate the association, further evidence is needed before any recommendations can be made.

The researchers said future vaccine and immunology studies should consider menstrual cycle phase as an important variable.

Type 2 diabetes is rising among women in their 20s in England, with experts warning of a worrying increase since the Covid pandemic.

The rise has largely occurred since the pandemic and is closely linked to increasing obesity levels, according to an analysis of NHS data from 2011 to 2024.

The condition is serious and can lead to severe complications, including heart attacks and strokes.

Diabetes UK said it tends to be more aggressive when it develops at a younger age, raising the risk of serious complications and shorter lives.

The charity said early diagnosis is vital for people at risk to help prevent long-term organ damage.

The condition can sometimes be put into remission through healthy eating and maintaining a healthy weight, without relying on glucose-lowering medicines.

Researchers said newer obesity treatments, including GLP-1 injections and pills, may also help alongside healthy eating advice.

About 90 per cent of the six million people living with diabetes in the UK have type 2 rather than type 1, or insulin-dependent, diabetes.

The risk increases with age, and older adults still account for the largest proportion of new diagnoses.

However, NHS England estimates suggest that 12,000 people under 30 are now living with the condition, with women making up more of the younger cases.

Common symptoms include feeling very tired, urinating more often than usual and feeling thirsty all the time.

Researchers at Imperial College London analysed NHS data from the English National Diabetes Audit to examine trends over time.

The figures showed that cases are falling slightly among older adults, who still account for the largest share of new diagnoses.

However, diagnoses are increasing rapidly among people under 40, with some of the sharpest rises among women aged 20 to 29.

Body mass index at diagnosis has also been rising more quickly among younger adults than in older age groups, pointing to obesity as a likely factor.

Body mass index, or BMI, is a measure of obesity based on a person’s height and weight.

Separate NHS data for England shows obesity rates have worsened since the pandemic, with nearly one in three people now affected.

The largest increases over time have been among younger adults.

Between 2019 and 2025, new obesity cases rose by 16 per cent among people aged 20 to 29 and by almost 20 per cent among those aged 30 to 39.

Rates fell among adults aged 60 to 79 over the same period.

Lead researcher Dr Shivani Misra said: “We think that the earlier onset of severe obesity in young people is increasing their type 2 diabetes risk in early life and pulling down their age at diagnosis.

“This is really worrying given the poor health outcomes from early-onset type 2 diabetes.”

Misra said much of the discussion about rising rates among younger people had focused on ethnic minority groups, which have traditionally been considered at higher risk.

However, the new data suggests the trend extends well beyond those communities.

She added: “We now see that incidence is also increasing in white populations too, showing that this public health challenge is broadening across the generations.”

Diabetes UK said better follow-up care is also needed for mums-to-be diagnosed with gestational diabetes during pregnancy because it raises the risk of developing type 2 diabetes after childbirth.

Some studies have suggested that the risk of developing the condition may also increase after being ill with Covid, although the evidence remains inconclusive.

Women with PMOS may have a fourfold higher risk of heart disease, new research suggests.

PMOS, or polyendocrine metabolic ovarian syndrome, is a complex hormone condition affecting about one in eight women in the UK.

It can affect hair growth, periods, fertility and mood.

US researchers analysed health insurance data collected between 2000 and 2022 from 413,450 women with PMOS aged 18 to 50.

They compared the group with more than two million women who did not have the condition.

The study found that women with PMOS had a fourfold higher risk of heart disease than those without it.

The increased risk remained after researchers accounted for other factors linked to heart disease, including high blood pressure and diabetes.

International PMOS guidelines recommend screening all patients for heart disease risk at diagnosis and providing advice on lifestyle changes.

However, researchers said patient surveys “report delayed diagnosis and substantial dissatisfaction with counselling regarding long-term comorbidities”.

Comorbidities are additional health conditions that occur alongside a main condition.

Researchers added: “Our results emphasise the need for physician education and patient awareness of cardiovascular disease risk to improve implementation of early preventive interventions.”

The team called for further studies following women through menopause, when periods stop.

They also called for research into whether treatments such as GLP-1 drugs or hormonal contraception affect the link between PMOS and heart disease.

GLP-1 drugs are medicines used for conditions including type 2 diabetes and weight management.

The main symptoms of PMOS include irregular periods or long gaps between periods, excessive hair growth or hair loss, weight gain or difficulty losing weight, and difficulty becoming pregnant.

Other symptoms include depression or anxiety, oily skin and acne, tiredness, and thick, dark patches of skin on the neck or armpits, known as acanthosis nigricans.

Until recently, PMOS was known as polycystic ovarian syndrome, or PCOS.

Experts called for the name change after highlighting that misunderstandings about cysts and too much focus on the ovaries were delaying diagnoses.

Earlier this month, the National Institute for Health and Care Excellence said women with PMOS should be seen by health professionals annually to monitor symptoms and manage long-term risks.

NICE said the condition affects between three and four million women in the UK but is “frequently underdiagnosed and inconsistently managed”.