People who have recovered from COVID-19 could return to the general population to help reduce the rate of transmission of the causative SARS-CoV-2 coronavirus, according to a modeling paper published in Nature Medicine. These people, who would need to be identified through antibody tests, could help in the development of ‘shield immunity’ within the wider community.
Without a reliable vaccine or treatment for COVID-19, current public-health strategies for dealing with the pandemic can be divided largely into two approaches: mitigation and suppression. Both strategies aim to reduce new SARS-CoV-2 infections by limiting the amount of human-to-human contact, but this can have long-term negative economic and social impacts.
Joshua Weitz and colleagues developed and analyzed an epidemiological model for reducing SARS-CoV-2 transmission. Their approach relies on the use of serological — or antibody — tests to identify people who have recovered from COVID-19. The model assumes that recovered people will be virus negative, will have protective antibodies to SARS-CoV-2 and will be able to interact safely with both susceptible and infectious people. These recovered people could then return to the general population and increase their interactions relative to other individuals. The authors suggest that this could build ‘shield immunity’ within the population by increasing interactions between recovered people and decreasing interactions between people with an unknown status.
The authors studied the impact of ‘shield immunity’ on a model population of 10 million under two scenarios: high transmission, with an R0 of 2.33, and low transmission, with an R0 of 1.57. The R0 value represents the number of cases an infected person is likely to cause while they are infectious in an otherwise susceptible population. The authors assessed the impact of intermediate shielding — when a recovered person substitutes for an additional two interactions — and enhanced shielding — when they substitute for an additional twenty interactions. In a high-transmission scenario, 71,000 deaths were predicted, but this decreased to 58,000 deaths under intermediate shielding and to 20,000 under enhanced shielding. In a low-transmission scenario, 50,000 deaths were predicted, while 34,000 and 8,400 were forecast under intermediate shielding and enhanced shielding, respectively. The model also shows that shielding can be used in concert with social distancing to reduce interactions while enabling recovered individuals to return to activities.
The authors caution that the baseline model assumes that a recovered person’s immunity will last one year or more, though they find robust results when immunity lasts four months or more. The duration of immunity is currently unknown. The authors also emphasize the need for accurate, population-wide serological testing to support any public-health interventions.
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