Deliberately infecting volunteers with COVID. Closely followed by the laters thinking on herd immunity
A vaccine candidate that has proven to be safe in initial trials
Carefully selected healthy adult volunteers, 90 people,18 – 30
Exposed to the virus in a safe and controlled environment
Closely monitor the effect on volunteers 24 hours
See exactly how the vaccine works
Identify any side effects
Controlled doses of virus
Virus characterisation study
Discover the smallest amount of virus it takes to
cause infection
Accelerate vaccine development
Monitored for up to a year after
Potentially saving thousands of lives
Controlled entrance to the facility
Decontamination of waste
A dedicated laboratory for carrying out tests
All the air leaving the unit is cleaned
Malaria, typhoid, cholera, norovirus and flu
John Hunter
James Phips
Robert Kock
Barry Marshall
January with results expected by May 2021.
Herd Immunity and Implications for SARS-CoV-2 Control (JAMA, 19th October 2020, Yale)
60% alcohol
Indirect protection
Community immunity
Community protection
Protection of susceptible individuals
Natural infection or through vaccination
Herd Immunity Threshold
The proportion of individuals in a population, having acquired immunity, can no longer participate in the chain of transmission
Threshold
Current outbreaks will extinguish
Herd immunity threshold depends on the basic reproduction number
Measles, will have a high R0 = 12-18
SARS-CoV-2 R0 = 2 to 3
Expected to range between 50% and 67% in the absence of any interventions
Duration of Protection
Durability of immune memory is a critical factor
Determining population-level protection and sustaining herd immunity
Measles, varicella, rubella
Seasonal coronaviruses
Pool of susceptible individuals increases again
Periodic vaccination can maintain herd immunity
Role of Heterogeneity
Nominal herd immunity thresholds assume random mixing between individuals
T-Cell Cross-reactivity
Cross-reactivity with other coronaviruses may confer relative protection
Less clear about sterilizing immunity
Host could not carry nor transmit infection …
as opposed to reducing the severity of illness
Infection-Based Herd Immunity as Policy
Letting the low-risk groups become infected
Sequestering or guarding susceptible groups
Low infection fatality ratios
Full guarding impractical
Zika in Salvador, Brazil
Sweden, until late March
Seroprevalence in Stockholm, 8% in April 2020
Similar to Geneva, Barcelona
United States
330 million
World Health Organization estimates, IFR = 0.5%
198 million to reach a herd immunity threshold of 60%
Conclusions
Infection-induced herd immunity is not realistic at this point
Vaccination programs must be efficient and broadly adopted
