The return of schools and the arrival of autumn means common colds and other respiratory infections are firmly on the rise, spreading coughs and sneezes, more severe illnesses, and prompting some to report their worst colds ever.

According to Public Health England, there is no particularly nasty new virus doing the rounds, but as cases rise, experts warn that people can expect more frequent infections and more serious symptoms now the UK is emerging from lockdown.

Common colds and other respiratory tract infections tend to ramp up in September when the schools go back and autumn arrives, but after 18 months of social distancing and mask wearing, many people are thought to have weaker immune defences to protect themselves against the onslaught of respiratory viruses.

With reduced immunity across the board, people may fall ill with viruses they would normally have fended off with little trouble, or develop co-infections that make them feel more poorly. This is particularly likely if the viruses that have been kept at bay by anti-Covid measures all bounce back at once.

“We don’t know what we’re going to see with common colds this season,” said Prof Ronald Eccles, former director of the Common Cold Centre at Cardiff University. “We’re seeing this increase now, but the whole system has been knocked out of kilter by the fact that we’ve been socially distancing and wearing masks, and children have not had that immunity over the past year or so.”

Common colds are caused by a huge range of viruses, including 100 different rhinoviruses, respiratory syncytial virus (RSV), adenoviruses, four types of non-Covid coronavirus, influenza and parainfluenza. Normally occasional exposure to the viruses, particularly in the winter, tops up our immunity to them.

Other countries have already seen surges in common colds after lockdowns. In Hong Kong, schools and nurseries witnessed an explosion in common colds when they reopened last autumn. It is not the only place this has happened. “In New Zealand … respiratory non-Covid illness is occurring earlier, is affecting more people – especially spread through schoolchildren and nurseries – and is more severe, most likely due to waning population immunity to them,” said Prof Stephen Holgate at Southampton general hospital.

“This is already happening in the UK, especially with RSV and childhood hospital and intensive care admissions, with the RSV occurring one to two months earlier than normal,” he added.

Public Health England surveillance released on Thursday showed that the number of people coming forward with common colds and other respiratory infections continues to rise, particularly among the under-15s, though cases are rising in older people too.

“It could well be that now common colds are resurging, because of the decline in social distancing and mask wearing, that they are bouncing back and the respiratory tract has not had enough recent experience of respiratory infections to be able to mount that strong first line defence,” said Prof Peter Openshaw, at Imperial College London.

In July, the Academy of Medical Sciences warned that a host of respiratory viruses, including flu and RSV, could bounce back in the winter and add to the strain of Covid on the NHS. As those infections ramp up, it will be crucial for people to have swift Covid tests if they develop symptoms, said Dr Gary Howsam, vice-chair of the Royal College of GPs.

“After more than a year during which most people apart from frontline staff have had limited contact with others, it’s inevitable that as we spend more time mixing with other people, we are starting to see a major resurgence of illnesses like common colds, flu and stomach bugs, alongside the Covid-19 virus in the community,” he said.

“Symptoms of Covid-19 can often be similar to symptoms of other illnesses such as the common cold. So it’s important that if a patient does experience symptoms of Covid-19, such as a high temperature, a new continuous cough, or a loss or change to sense of smell or taste that they get a PCR test immediately, and on receiving a positive result, self-isolate for the required period of time.”

Dr Yvonne Doyle, medical director at Public Health England, said: “As we head into winter, there are steps we can all take to keep ourselves healthy and reduce the burden on the NHS. Hand washing, ventilation and mask wearing aren’t only important for Covid-19, they keep other bugs at bay too. Try to stay at home if you are unwell.”