//First viral human challenge study

First viral human challenge study

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


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%


Infection-induced herd immunity is not realistic at this point

Vaccination programs must be efficient and broadly adopted