The massive whooping cough outbreak sweeping Scotland is likely to get worse over the coming months, an expert has warned.

Dr Sam Ghebrehewet, head of immunisation and vaccination at Public Health Scotland (PHS), said infections would probably not peak until the autumn.

It comes days after PHS revealed that a total of 2,232 laboratory-confirmed cases of pertussis - better known as whooping cough - had been detected in Scotland up to May 13.

That compares to just 73 in the whole of 2023, and far exceeds the numbers reported in the last significant outbreak in 2012 and 2013, when 3,084 cases were detected over a two-year period.

In England, five young infants - all under three months old - died as a result of whooping cough between January and March this year.

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There have been no deaths to date in Scotland, but expectant mothers have been urged come forward for vaccination if they have not already had the jag during pregnancy.

Speaking to the BBC's Sunday Show about the outbreak, Dr Ghebrehewet said: "It is likely to get worse.

"The cycles we have seen over the last 10 years is usually that it starts in the first [quarter of] the calendar year - January, February, March.

"It goes higher and higher and it doesn't peak until the third quarter, so I think it has a long way to go as far as we understand from the previous years.

"I think it's going to increase rather than decrease at this stage."

Women are usually eligible for vaccination at between 16 and 32 weeks gestation, but Dr Ghebrehewet stressed that it was "not too late" to come forward even if 32 weeks had passed.

The four-in-one inoculation during pregnancy enables mothers to pass antibodies onto their developing baby in the womb, so that they have protection against pertussis - and other infectious diseases - for the first few months after birth, until they are old enough for vaccination.

Babies are offered pertussis vaccination at eight, 12, and 16 weeks, with a booster from three years and four months old.

Falling vaccination uptake among mothers and infants, coupled with a decline in exposure due to social distancing during the pandemic, have been blamed for the scale of the current outbreak.

The Herald: Babies under one who have not been vaccinated are most at riskBabies under one who have not been vaccinated are most at risk (Image: Getty)

Figures for England show that just 59.3% of pregnant women were vaccinated against whooping cough between October and December last year, down by almost 16% compared to 2016/17.

In London, coverage was just 36.8%.

It is unclear what uptake in pregnancy has been in Scotland as the figures are held by individual health boards, rather than being collated centrally by PHS - as statistics on childhood immunisations are.

On Thursday, First Minister John Swinney was pressed in parliament for figures showing the proportion of pregnant women who are taking up the vaccine in Scotland.

He promised to seek specific information for the vaccination programme, but added: “Generally the uptake [of vaccines] is moving in the right direction which we would want to see.”

Although still high, uptake of the pertussis jag for babies in Scotland has been falling.

Coverage among one-year-olds went from 98% in December 2014 to just under 95% by September 2023.

Among the mainland NHS boards, coverage was lowest in the Scottish Borders, at 91.5%, and 91.8% in the Highland region.

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Whooping cough is caused by the bacteria, Bordetella pertussis. 

Before the pertussis vaccine was added the routine schedule of childhood vaccinations in Britain in the 1950s, there were around 150,000 known cases per year and more than 300 deaths.

Unvaccinated babies under one are still most at risk.

Symptoms resemble the common cold in the early stages, such as a runny nose, and progress to prolonged bouts of coughing sometimes characterised by a "whoop" sound and vomiting.

A resurgence in whooping cough infections in recent decades has been partly attributed to a change in the type of vaccine used.

The Herald: Cases of pertussis (whooping cough) all but disappeared during the pandemicCases of pertussis (whooping cough) all but disappeared during the pandemic (Image: PHS)

The first generation of vaccines used from the 1950s to the 1970s was "whole cell", meaning that it was formulated using an inactivated form of the pertussis bacteria.

This produced a strong and durable protective immunity, but by the early 1970s the vaccine was associated in some cases with neurological complications.

It was subsequently replaced by acellular vaccines which still retain good protection against serious disease, but immunity against infection wanes much more quickly.

Dr Ghebrehewet said this had resulted in the "cyclical" pattern of whooping cough outbreaks seen over the past 10 to 20 years.

He said: "[Whooping cough] comes and goes every three to four years, and particularly I think this is related to the vaccine effectiveness in terms of how long the protection lasts.

"We understand now that the vaccine protection lasts about seven to 10 years, so children are vaccinated at the age of two, three, four months, and the last of these are given at the age of three to four [years], so after seven years the vaccine protection is waning.

"Effectiveness is waning in terms of infections, but effectiveness remains really high in terms of serious infection and hospital admissions and mortality."