Fewer people having sex during lockdowns and disruption to health services contributed to a steep drop in STI diagnoses last year, experts say.
The British Association for Sexual Health and HIV warned that the latest figures could represent "the tip of the iceberg".
Public Health England data shows 977 STIs were diagnosed in Northumberland in 2020 – 36% fewer than the year before.
It meant 302 in every 100,000 people in the area were infected with potentially life-changing diseases including syphilis, gonorrhea and chlamydia.
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However, that rate was down from 2019, when 470 in 100,000 people in Northumberland were diagnosed with an STI.
The most common infection in the area was chlamydia with 591 cases found in 2020. A further 87 gonorrhoea cases were diagnosed, as well as four of syphilis, 114 of genital herpes and 121 of genital warts.
Dr John McSorley, president of BASHH, said the national drop in diagnoses highlighted the "stark and concerning" impact Covid-19 has had on sexual health services.
He added: "Whilst a drop in the number of new infections appears positive, it is important to remember that England entered the Covid pandemic with the highest rates of some STIs since the Second World War.
"This data therefore likely represents the tip of the iceberg. STIs haven't gone away, chains of infections haven't been broken."
He urged people to come forward for testing, saying sexually transmitted infections could have life-changing consequences.
Dr Katy Sinka, from 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.”
The national drop reflects a combination of reduced STI testing as a result of pandemic-influenced disruption to sexual health services and changes in sexual behaviour since March 2020, according to a PHE report.