For the first time, older adults in Scotland and pregnant women will be offered a vaccine for Respiratory syncytial virus (RSV).
Nearly all of us with have had the infection in our lifetimes - probably several times - but for some people it can cause severe illness.
So why is rollout starting now, and what can we expect?
What is RSV?
RSV is a common and highly infectious respiratory virus that affects the breathing system.
It generally causes mild illness with cold-like symptoms, such as a runny nose, cough, fever, or decrease in appetite.
Most people recover after two to three weeks, but RSV can cause severe illness in some people, including pneumonia and bronchiolitis (an inflammation of the small airways in the lungs), leading to complications and hospitalisation.
Who does it affect?
The vast majority of children are infected by RSV before the age of two, and most people will be reinfected several times in their lifetime.
In adults and healthy, older children symptoms are mild and cold-like.
However, the virus can pose a higher risk to babies under 12 months old, premature infants, elderly people, patients with heart and lung diseases, and people who are immunocompromised.
In most years, the virus tends to peak toward the end of each year, from around October.
There were more than 4000 acute hospital admissions last winter in Scotland for patients with RSV.
It also causes hundreds of deaths in Scotland each winter, mostly among over-75s.
Public health experts anticipate that a high uptake of the vaccine will substantially reduce RSV-related hospital pressures on the NHS over winter.
Why are we vaccinating against it now?
Until now, RSV vaccines have not been available.
The Medicines and Health products Regulatory Agency (MHRA) authorised the Abyrsvo vaccine - manufactured by Pfizer - for use in older adults and pregnant women in late 2023.
A separate RSV vaccine - GSK's Arexvy - was also approved last year for the prevention of lower respiratory tract disease (LRTD) caused by RSV in adults 60 years and older.
The NHS rollout will use Abyrsvo.
Who is getting vaccinated?
For older adults, vaccinations are being offered to people aged 75 to 79 initially.
Work is ongoing to determine whether it is cost effective for the NHS to offer the vaccine to younger and older age groups.
It will also be offered to all pregnant women from 28 weeks gestation.
What's the evidence?
In clinical trials, Abrysvo was given to more than 17,000 adults over the age of 60 and in more than 4,000 pregnant women.
It was shown to reduce by 70% the risk of severe respiratory tract infections in newborns up to six months old - the period they are most vulnerable - if given to their mothers in later pregnancy.
It was approximately 73% to 79% effective in preventing RSV-associated hospitalisation or emergency department encounters in adults 60 and older.
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