Scientists identify when COVID-19 is most transmissible before and after symptoms

Fitness & Health:

Cases of COVID-19 are still high in the UK, especially among young people. With the reopening of schools and universities looming, a spike in cases could be imminent, according to experts. Reports over the past year have raised concerns that people who appear healthy may be major contributors to the spread of the virus, prompting scientists to identify some of the primary drivers of COVID-19. New research has determined when transmission of the virus is most likely before or after symptoms appear.

The study, led by the Boston University School of Public Health, found that people who test positive for the virus are most contagious two days before and three days after they develop symptoms.

Doctor Leonardo Martinez, assistant professor of epidemiology and co-author of the study, said: “In previous studies, viral load has been used as an indirect measure of transmission.

“We wanted to see if results from these past studies, which show that COVID cases are most transmissible in a few days before and after symptom onset, could be confirmed by looking at secondary cases among close contact.”


For their study, Martinez and colleagues studied transmission rates among approximately 9,000 close contacts, defined as household contacts, co-workers, people in hospital settings and riders in shared vehicles.

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Doctor Martinez noted: “Our results suggest that the timing of exposure relative to primary-case symptoms is important for transmission, and this understanding provides further evidence that rapid testing and quarantine after someone is feeling sick is a critical step to control epidemic.

“This study further emphasises the need for vaccination which reduces clinical severity among people that develop COVID-19.”

Like adults, children and adolescents are susceptible to COVID-19 infections. Although a small number of children may develop severe illness, they are more likely to be asymptomatic or have mild symptoms compared to adults.

Studies have compared the ability of children to transmit the virus with that of adults, however, data remains largely inconclusive.

Speaking on the Today programme on Radio 4, Professor Devi Sridhar, chair in global public health at the University of Edinburgh, suggested vaccine advisers were being too cautious.

He said: “They’re waiting and watching, and I guess the issue with a pandemic is that waiting and watching cost time. Time is the currency that matters, because it’s not like we can wait and watch and in six months say: OK, it’s safe, let’s vaccinate.”

Alicia Demirjian, incident director for COVID-19 at Public Health England, noted: “Cases of COVID-19 are still high, especially among young people.

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“If you have COVID-19 symptoms, please do not go out – stay at home and get a PCR test as soon as possible. If you are heading to a festival or other events this Bank Holiday weekend, to protect your friends and others we encourage you take a free rapid test before your leave to check that you do not have the virus.”

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