Public Health England said cases reported in the worst outbreak for two decades fell to 188 in May, down from 299 in April. A total of 1,168 cases have been reported so far this year.
Estimates suggest around 56,000 children aged 10-16 have received a first dose of MMR in the catch-up campaign, which began in April. This is about half of those set to be targeted.
A similar number have received a second dose of MMR.
Officials said the drop in cases may show the campaign is starting to have an impact, although it was too early to confirm this.
GPs have ordered 200,000 extra MMR doses in the programme so far. Officials are aiming to protect 95% of all children aged 10-16.
Berkshire GP Dr George Kassianos, RCGP lead on immunisation, said: 'I am delighted to hear so many children have been vaccinated but every child that remains not immunised must be offered the vaccine. We need to achieve over 95% immunisation rate before we can start thinking of relaxing the campaign.'
Professor David Salisbury, DH director of immunisation, said: 'It's encouraging that GPs have taken up the challenge wholeheartedly but we now need to make sure that all children at risk are vaccinated.'
Dr Mary Ramsay, head of immunisation at Public Health England, said: 'Thanks to the hard work of local health teams we are making good progress towards the 95% target, but there still remains a large number of 10- 16-year-olds, together with many younger children and adults, who are under-vaccinated.'
The north west of England continues to be most affected by the outbreak, although numbers are high in the north east and increasing in London.
One in five cases this year, around 230, have been admitted to hospital.