The Economist - USA (2020-11-28)

TheEconomistNovember 28th 2020 69

1

O

n november 23rd, for the third Mon-

day in a row, the results of an anti-

covid vaccine were announced. This time

the protagonists were AstraZeneca, a Brit-

ish-Swedish pharmaceutical company,

and Oxford University. They reported their

vaccine to be 70% effective. But doubts

have since arisen about the conduct of the

trials which arrived at this figure.

The consortium’s researchers estimat-

ed their vaccine’s efficacy from interim

data collected by trials in Britain and Brazil.

These involve more than 23,000 volun-

teers, half of whom have received the vac-

cine and the other half a placebo of one sort

or another. Like the previous offerings (one

from a partnership between Pfizer, a big

American pharma company, and BioN-

Tech, a small German one; the second from

another American firm, Moderna), the As-

traZeneca-Oxford inoculation is adminis-

tered in two jabs. Of those given it, none

has been admitted to hospital with co-

vid-19, nor suffered a severe case of it. The

researchers from AstraZeneca and Oxford

also say that their vaccine may reduce pas-

sage of the virus between people—a prop-

erty not yet established for either of the

other two. An ideal vaccine needs to break

the chain of transmission, as well as stop-

ping recipients from falling ill, so this

claim is important.

Half measures?

The consortium’s claim of 70% efficacy is,

however, complicated by concerns about a

slip-up during the trials which meant some

participants were given only a half-dose of

the vaccine to start with, followed by a full

one. Intriguingly, the efficacy of this ap-

proach seemed to be about 90%. But if that

is confirmed as correct, it follows that the

efficacy of the intended protocol is actually

only 62%. Further complications are that

the half-dose group was a small fraction of

the total, making statistical analysis less

robust, and that group members were un-

der 55, rather than fully representative of

the population.

Why a lower initial dose might yield a

stronger response is puzzling. The answer

may lie in how the AstraZeneca-Oxford

vaccine delivers genetic material from the

coronavirus into the recipient’s body, in or-

der to stimulate that person’s immune sys-

tem. It uses what is known as a viral vector

to carry this material, a substance called

rnathat is a cousin of dna. The other two

vaccines deliver rnaas a “naked” mole-

cule, wrapped up in a fatty bubble.

Before the trials began, some research-

ers had worried that going down the viral-

vector route might cause people to develop

immunity to the vector as well as to the co-

ronavirus proteins which the rnapayload

would encourage that person’s cells to

make. The findings from the accidental

subgroup hint that anti-vector immunity

may, indeed, be happening, causing a large

first dose to prime the body to develop anti-

bodies to the vector. These would then at-

tack the second-dose vectors before they

could do their job. A smaller first dose

might diminish this priming effect—ex-

plaining the better efficacy of unequal

doses. The researchers expect to gather

The coronavirus pandemic

Another covid-19 vaccine joins the party

But there are doubts about the data describing its efficacy

Science & technology

70 A cheap,solar-poweredautoclave

72 Fossilsharknurseries

72 Carseatsandbirthrates

Also in this section

74 The death of Arecibo