Symptoms of the monkeypox virus do not appear until up to four days after transmission, scientists have discovered.
UK Health Security Agency (UKHSA) researchers estimated that more than half (53%) of transmission occurs before symptoms are known to person with the virus.
The findings showed that four days was the longest time that the virus was detected before symptoms appeared.
The breakthrough evidence indicates many infections cannot be prevented by asking people to isolate themselves when they notice symptoms.
Monkeypox symptoms vary between people but include rash, fever, chills, swollen lymph nodes, exhaustion and headache.
The findings also limit the effectiveness of contact tracing, because contacts may have passed the virus on to others before they are traced.
The study concluded that people would need to isolate for 16 to 23 days to detect 95% of those with a potential infection.
In September, the UKHSA announced that second doses of the smallpox vaccine are being offered to those people at highest risk from monkeypox.
More than 45,000 people have received a dose of the vaccine, including more than 40,000 gay, bisexual and other men who have sex with men.
These groups have been found to be at the highest risk of exposure to monkeypox.
UKHSA figures show that, as of 24 October, there have been 3,548 confirmed and 150 “highly probable” monkeypox cases detected in the UK.
Cases peaked at more than 60 per day in mid-July, but have been falling since, with fewer than 15 cases per day on average in early September.
Vaccination "crucial" against virus - study director
Dr Nachi Arunachalam, monkeypox incident director at the UK Health Security Agency, said: "There is still more work needed to understand pre-symptomatic and asymptomatic infections and what that might mean for future policies and management of the monkeypox outbreak.
“Whilst we continue to see fewer cases reported in the UK, it remains vital people are alert to the risk monkeypox poses and take action to protect themselves and others.
“Vaccination plays a crucial role in this so I would encourage those at highest risk to come forward and get your first dose.”
The study, published in the British Medical Journal (BMJ), included 2,746 people who tested positive for monkeypox in the UK between May 6 and August 1.
Researchers were examining the time it took from when first symptoms occurred in the first patient to when symptoms developed in a second patient.
They also looked at the incubation period - which is the time from exposure to the virus to when symptoms appeared.
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