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The Iow level of current COVID-19 infections in Europę presents a challenge for Phase ll/lll trials seeking to prove the efficacy of vaccine candidates

Notę:    'Assumes 59,000 people infected on 24 May 2020. and 93% of U.K. population remain susceptible to infection

Source: ONS U.K.; WHO; L E K. analysis 6

Cumulative probability of COVID-19 infection in U.K. over time (illustrative)*

Percent

•    Phase ll/lll trials have begun for a number of potential vaccine candidates across Europę and in the U.S.

•    In Europę, the estimated current level of infection is Iow. The ONS estimates that c.59,000 people in the U.K. - 0.1% of the population - had COVID-19 on an average day between 17 and 30 May 2020. This presents a challenge for proving vaccine efficacy, as the likelihood that a subject is exposed to the SARS-CoV-2 virus naturally is Iow

•    For example, if the reproduction ratę in the U.K. were to continue at 0.75, then only 22 people per 10,000 would be expected to have caught the disease by September 2020, or 155 people per 10,000 if R increased to 1.2. These proportions may be too Iow to prove that a vaccine has been effective

•    One potential solution to this problem being discussed by scientists is to deliberately expose subjects to the virus in so-called “challenger trials”. There is precedent for challenger trials (particularly for influenza), but they raise considerable medical ethics concerns. In the case of COVID-19, the virus is not yet well understood and there is no definitive cure. Nevertheless, the WHO has published guidance on the ethical considerations of such a trial

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