- Scientists found out what makes people contagious with covid.
- In a rare transmission study, scientists found out people with higher viral load are spreading it.
- The transmission is found to be more in household contacts than anywhere else.
- Also the time of onset of covid in such cases are 6-7 days.
- This indicates high viral load people with shorter incubation period are the superspreaders.
A study conducted in 300 covid infected people reveals that the amount of SARS-CoV-2 in a person’s body is a major factor in determining whether they are likely to transmit the virus to others, says a Lancet report.
Why people become superspreaders?
Most people with COVID-19 do not give it to anyone else, but some become ‘superspreaders’. To understand why, Michael Marks at the London School of Hygiene & Tropical Medicine and his colleagues monitored 282 people, deemed ‘index cases’, who had recently developed mild symptoms of COVID-19. The team also monitored 753 people who lived with, cared for or otherwise had close contact with the index cases (M. Marks et al. Lancet Infect. Dis. https://doi.org/10.1016/S1473-3099(20)30985-3; 2021).
- Only one-third of the index cases transmitted the virus to a close contact.
- Those with a relatively high ‘viral load’, a measure of the amount of virus in the body, were much more likely to pass on the virus than were those with a low viral load.
- Index cases were no more likely to transmit the virus if they had a cough than if they didn’t.
Assessing Factors Affecting Transmission
Scarce data are available on what variables affect the risk of transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the development of symptomatic COVID-19, and, particularly, the relationship with viral load. We aimed to analyse data from linked index cases of COVID-19 and their contacts to explore factors associated with transmission of SARS-CoV-2.
How was it done?
In this cohort study, patients were recruited as part of a randomised controlled trial done between March 17 and April 28, 2020, that aimed to assess if hydroxychloroquine reduced transmission of SARS-CoV-2.
- Patients with COVID-19 and their contacts were identified by use of the electronic registry of the Epidemiological Surveillance Emergency Service of Catalonia (Spain).
- Patients with COVID-19 included in our analysis were aged 18 years or older, not hospitalised, had quantitative PCR results available at baseline, had mild symptom onset within 5 days before enrolment, and had no reported symptoms of SARS-CoV-2 infections in their accommodation or workplace within the 14 days before enrolment.
- Contacts included were adults with a recent history of exposure and absence of COVID-19-like symptoms within the 7 days preceding enrolment.
- Viral load of contacts, measured by quantitative PCR from a nasopharyngeal swab, was assessed at enrolment, at day 14, and whenever the participant reported COVID-19-like symptoms.
Key Yardsticks of Assessments
- We assessed risk of transmission and developing symptomatic disease and incubation dynamics using regression analysis.
- We assessed the relationship of viral load and characteristics of cases (age, sex, number of days from reported symptom onset, and presence or absence of fever, cough, dyspnoea, rhinitis, and anosmia) and associations between risk of transmission and characteristics of the index case and contacts.
Key Findings
- We identified 314 patients with COVID-19, with 282 (90%) having at least one contact (753 contacts in total), resulting in 282 clusters.
- 90 (32%) of 282 clusters had at least one transmission event.
- The secondary attack rate was 17% (125 of 753 contacts), with a variation from 12% when the index case had a viral load lower than 1 × 106 copies per mL to 24% when the index case had a viral load of 1 × 1010 copies per mL or higher (adjusted odds ratio per log10 increase in viral load 1·3, 95% CI 1·1–1·5).
- Increased risk of transmission was also associated with household contact (3·0, 1·59–5·65) and age of the contact (per year: 1·02, 1·01–1·04). 449 contacts had a positive PCR result at baseline. 28 (6%) of 449 contacts had symptoms at the first visit.
- Of 421 contacts who were asymptomatic at the first visit, 181 (43%) developed symptomatic COVID-19, with a variation from approximately 38% in contacts with an initial viral load lower than 1 × 107 copies per mL to greater than 66% for those with an initial viral load of 1 × 1010 copies per mL or higher (hazard ratio per log10 increase in viral load 1·12, 95% CI 1·05–1·20; p=0·0006).
- Time to onset of symptomatic disease decreased from a median of 7 days (IQR 5–10) for individuals with an initial viral load lower than 1 × 107 copies per mL to 6 days (4–8) for those with an initial viral load between 1 × 107 and 1 × 109 copies per mL, and 5 days (3–8) for those with an initial viral load higher than 1 × 109 copies per mL.
Tracing High Viral Load Contacts the Key
In our study, the viral load of index cases was a leading driver of SARS-CoV-2 transmission. The risk of symptomatic COVID-19 was strongly associated with the viral load of contacts at baseline and shortened the incubation time of COVID-19 in a dose-dependent manner
The findings suggest that tracing the contacts of people with high viral loads is especially important, the authors say.
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Source: Nature
