Diagnoses of sexually-transmitted infections in England were down by almost a third last year, according to the latest figures.
The drop is thought to be due to a combination of less testing during the disruption to sexual health services amid the pandemic as well as changes in behaviour as the country went into lockdown.
People are now being warned to be careful not to "swap social distancing for an STI (sexually-transmitted infection)".
Public Health England said diagnoses decreased by 32% last year compared with 2019, but added that despite the fall their data shows that diagnoses remain high overall.
Last year, services to diagnose infections were scaled up to take the form of phone and internet consultations, and PHE said 317,901 STIs were diagnosed.
Consultations at sexual health services in 2020 decreased by 10% on 2019, with the biggest drop in face-to-face consultations.
These were down by 35% on 2019, PHE said, while internet consultations doubled over the same time period.
The health body also said there was a 25% fall in sexual health screening - tests for chlamydia, gonorrhoea, syphilis or HIV.
PHE said infections that require an in-person assessment, like genital warts and herpes, saw a greater drop in diagnoses. They fell by 46% and 40% respectively.
Others that can be diagnosed using self-sampling kits and an internet consultation fell by less, with chlamydia and gonorrhoea dropping by 29% and 20% respectively.
PHE said the highest rates of STI diagnoses were still seen in people aged 15 to 24, people of black ethnicity, and gay, bisexual and other men who have sex with men.
These trends are the same as in previous years, they added.
Dr Katy Sinka, head of the sexually-transmitted infections section at PHE, said: "No-one wants to swap social distancing for an STI and, as we enjoy the fact that national Covid-19 restrictions have lifted, it's important that we continue to look after our sexual health and wellbeing.
"If you are having sex with new or casual partners, use a condom and get tested - STIs can pose serious consequences to your own health and that of your current or future sexual partners."
Debbie Laycock, head of policy at HIV and sexual health charity Terrence Higgins Trust, welcomed the "significant drop" as "one unexpected good news story from the coronavirus pandemic".
She insisted that online testing "must now be maintained and expanded with greater consistency across the country".
She added: "It is good news that we've finally seen a significant drop in STI rates in England, but we need to 'build back better' after Covid in terms of our STI response and capitalise on this once-in-a-generation scenario.
"That includes investment in our sexual health services and ensuring the Government's sexual and reproductive health strategy, due out later this year, is properly funded and ambitious enough to improve the nation's sexual health."
David Fothergill, chairman of the Local Government Association's community wellbeing board, said there could be a "surge in demand (for tests) as life returns to normal" and called for investment in services.
He said: "While we have seen a decline in STIs, this also suggests that some people who were infected might not have sought advice during lockdown restrictions.
"Investing in councils' public health budgets now will help relieve pressure and save money for other public services, including the NHS and social care, by supporting people to live healthier, safer lives, as we look to build back better from the pandemic and level up the country."