As Britain enjoyed a summertime lull in COVID-19 cases, the nation’s attention turned to the end of pandemic-related restrictions and holidays in the sun, AP reported.

But scientists are warning the public not to be complacent, saying high levels of infection in the community are likely to lead to another spike in cases this fall.

The reason for their pessimism is the delta variant of COVID-19, now dominant throughout the UK.

Vaccines are less effective against this more transmissible variant, meaning Britain needs to achieve a much higher level of vaccination if it hopes to control the disease. About 60% of the UK population has been fully vaccinated.

“If you’re going to rely on the vaccines, OK, then vaccinate everybody,” said Ravi Gupta, a University of Cambridge professor who did some of the pioneering studies on the delta variant. “But they’ve done a half vaccination job and then they’ve opened everything up. And this is a recipe for … things not going well in the next few months.”

Prime Minister Boris Johnson said Britain’s successful vaccination program meant people were much less likely to get seriously ill or die from COVID-19.

But after a drop in confirmed new infections following July 19, cases have plateaued at an average of around 25,000 a day, more than 10 times higher than in early May.