Cases of measles and mumps in England are rising sharply, figures have shown amid fears that growing numbers of people are not getting immunised because of “dangerous” myths about vaccines.

Public health experts reacted with alarm to the increases and warned that misinformation about vaccination is leading to parents refusing to let their children have the MMR jab.

Of the 301 people diagnosed with measles between April and June, 266 were aged 15 years or over and had not been vaccinated, while nearly half of those who contracted mumps were unvaccinated.

“These stark rises in mumps and measles cases show that complacency about vaccines is misplaced and dangerous, which is why the NHS is taking action to boost take-up rates and tackle the fake news peddled on social media and elsewhere,” said Simon Stevens, the chief executive of NHS England.

“Parents have enough to worry about without misleading anti-vaxxers sowing seeds of doubt about vaccines, which remain the best chance we have of protecting our children from potentially deadly illnesses.”

The 301 cases of measles represented a 30% increase in three months, compared with 231 cases reported in January to April.

The spike continued the recent upward trend in the number of those getting the virus, which is highly infectious and can prove fatal. The totals recorded in both the first two quarters of this year were much higher than the 97 cases in October to December last year.

The figures, released on Friday by Public Health England, come 24 hours after the World Health Organization said measles had returned to the UK. As a result, the WHO removed the “measles-free” status it had previously awarded to Britain and three other countries – Greece, Albania and the Czech Republic.

NHS England’s chief executive, Simon Stevens: ‘Complacency about vaccines is misplaced and dangerous.’ Photograph: Stefan Rousseau/PA

There has been an even bigger rise in the number of people – especially university students – getting mumps. Between April and June, 2,028 people contracted the infection. That was more than double the 795 who did so in the previous quarter and a 12-fold increase on the 170 diagnoses seen in the last three months of 2018.

“The increase in mumps has been mostly driven by outbreaks in university students. Cases were reported across England, predominantly in young adults aged 15 to 34 years. Although it is normal to see mumps outbreaks in universities every few years, we are seeing a significant number of cases, the highest quarterly figure since 2009,” said Dr Mary Ramsay, PHE’s head of immunisation.

While the mumps component of the measles, mumps and rubella (MMR) vaccine is very effective at protecting young children, the immunity it confers can go down over time, she added. As a result, older teenagers and adults who received two doses of the vaccine in childhood can still get mumps, although this is generally mild compared with those who are unvaccinated, she explained.

“Coupled with the continued measles outbreaks, these figures clearly demonstrate the need for sustained high vaccination rates. We’re urging parents and their children, no matter how old they are, to check they’ve had two doses of MMR.

“Measles is easy to catch and can kill. Vaccines are there to stop the spread of disease and save lives. It’s never too late to protect yourself and others,” Ramsay said.

PHE insists its most recent annual attitudinal research into parental attitudes towards vaccines shows that their confidence in them remains high. The proportion of five-year-olds in England who have received the recommended first and second doses of MMR have remained steady, at around 95% and 87% respectively, in recent years.

The growing difficulty patients face in getting an appointment at a GP surgery is much more likely to explain the rises in measles and mumps diagnoses, PHE suggested. A report last year by the Royal Society for Public Health (RSPH) identified GP access as a key barrier to vaccine uptake.

Will Durant, the RSPH’s vaccines policy lead, said that – coming so soon after the WHO’s action – “the new figures on mumps as well as measles cases further confirm that something is going wrong in our attempts to guard against diseases that were once thought safely eradicated”.

He added: “There’s no silver bullet for reversing this worrying trend, but health authorities now need to push on to make the case for the MMR vaccine and vaccinations in general: that they are safe, effective and save lives.

“The growth of misinformation online is also a potential cause for concern. While addressing ‘anti-vaxxer’ content online is by no means the whole story when it comes to ensuring high vaccination coverage, we know that the online world can be a hotbed for negative content about vaccines, and social media platforms need to step up and take responsibility for addressing these concerns,” he added.