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First SARS-CoV-2 human challenge study shows an incubation period of just 2 days

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Dr Veena Aggarwal, Consultant Womens’ Health, CMD and Editor-in-Chief, IJCP Group & Medtalks Trustee, Dr KK’s Heart Care Foundation of India    04 February 2022

Results of the first SARS-CoV-2 human challenge study published on the preprint server Research Square from the UK show that symptoms of Covid-19 infection develop within two days of exposure and peak viral load occurs at 5 days.1

In this trial, 36 healthy participants, aged 18-29 years, unvaccinated and with no history of Covid-19 infection were enrolled. All participants were administered a very small dose of coronavirus via nasal drops. They were then quarantined for two weeks. The virus was the original virus strain before the detection of the alpha variant. All participants will be followed up for a year to monitor for any potential long-term effects.

Out of the 36 participants, 18 (53%) developed the infection. Sixteen (89%) of these developed symptoms of mild to moderate Covid-19 such as  runny nose, sneezing, sore throat, headaches, muscle/joint aches, tiredness and fever. Twelve (67%)  patients reported anosmia; except three, all of them recovered their sense of smell within 90 days. None of the infected persons developed severe disease or had evidence of lung involvement.

However, the incubation period i.e. the time between exposure to infection and appearance of symptoms was 42 hours i.e. within 2 days, which is much shorter than the currently estimated incubation period of 5-6 days, followed by a sharp rise in the viral load in the nose or throat. The peak viral load was noted at 5 days, but the high levels of the infectious virus continued to be detected even after nine days and even up to 12 days in some individuals.

The virus was detected earlier in the throat than in the nose; 40 hours vs 58 hours respectively. Peak levels of the virus also occurred earlier in the throat and were lower than in the nose suggesting a greater chance of viral shedding from the nose than the mouth. No correlation between the amount of viral shedding and symptom score was observed. High viral loads were noted even in asymptomatic infections.

Lateral flow tests were effective in detecting the presence of the infectious virus and positive lateral flow tests correlated with positive RT PCR tests.

This landmark trial was carried out in collaboration between the Imperial College London, the Vaccine Taskforce and Department of Health and Social Care (DHSC), hVIVO (part of Open Orphan plc.) and the Royal Free London NHS Foundation Trust.

The trial is the first to monitor people through the entire course of infection and provide insights into the timeline of the infection. According to a press release from the Imperial College London UK, Professor Christopher Chiu, Dept. of Infectious Disease and the Institute of Infection at Imperial College London and Chief Investigator on the trial, said: “Our study reveals some very interesting clinical insights, particularly around the short incubation period of the virus, extremely high viral shedding from the nose, as well as the utility of lateral flow tests, with potential implications for public health.”

It has shown that exposure to a single nasal droplet is enough to develop Covid-19. People became symptomatic within 2 days of exposure and the peak infectivity occurred five days into the infection.

These findings have public health significance. The high viral shedding from the nose underscores the need to wear well-fitting three-layered face masks that cover the nose and the mouth. Another key finding was the detection of the infectious virus at 10 days after exposure.  Hence, duration of isolation should be at least 10 days after the onset of symptoms.

Reference

  1. Killingley B, et al. Safety, tolerability and viral kinetics during SARS-CoV-2 human challenge. Research Square 2022; DOI: 10.21203/rs.3.rs-1121993/v1.

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