The Economist - USA (2021-02-06)

The Economist February 6th 2021 63

Science & technology

Making vaccines

Doing the do

N

ine vaccinesagainst covid-19 have al-

ready been approved in one jurisdic-

tion or another, with many more in various

stages of preparation. That this has hap-

pened within a year of the illness coming

to the world’s attention is remarkable. But

it is one thing to design and test vaccines. It

is another to make them at sufficient scale

to generate the billions of doses needed to

vaccinate the world’s population, and to do

so at such speed that the rate of inocula-

tion can outpace the spread and possible

mutation of the virus.

Broadly, there are two ways of making

antiviral vaccines. One, tried and trusted,

involves growing, in tanks called bioreac-

tors, cell cultures that act as hosts for vi-

ruses which are then used in one way or

another to make the vaccine in question.

Cells grown this way can be of many

types—insect, human kidney, monkey kid-

ney, hamster ovary—as can the resulting

vaccines. These may be weakened or killed

versions of the virus to be protected

against, or live viruses of a different and

less-dangerous sort that carry a gene or

two abstracted from the target virus, or

even just isolated target-viral proteins. The

point is that the vaccine should introduce

into the body, or induce that body to make,

something which the immune system can

learn to recognise and attack if the real tar-

get virus should ever turn up.

In with the new

The alternative method, developed recent-

ly and employed to make the mrnavac-

cines, such as those of Moderna and Pfizer,

that the pandemic has stimulated the in-

vention of, requires culturing cells only at

the beginning of the process. mrnais the

substance that carries instructions about

how to make a protein from a cell’s dnato

the molecular factories, known as ribo-

somes, which do the actual manufactur-

ing. In the case of covid-19, the instruc-

tions in question generate spike, a protein

found on the surfaces of particles of sars-

cov-2, the virus that causes this illness.

Suitably packaged and delivered, such

mrnacan induce some of the body cells of

the inoculee to turn out spike, which the

immune system then learns to recognise.

To make this type of vaccine you therefore

have to generate lots of the relevant mrna.

That process does indeed start with

cells, though they are bacterial cells, rather

than those of animals. But it does not end

with them. The bacteria used, normally a

well-understood species called E coli, have

spliced into them a dnaversion of the part

of the sars-cov-2 genome which describes

spike. (Confusingly, as is true of many vi-

ruses, sars-cov-2’s actual genes are made

of rna.) The bacteria are then allowed to

multiply for a few days before being bro-

ken open, their dnafiltered out, and the

dnaversions of the spike gene extracted as

what is known as a dnatemplate.

Once purified, this template is mixed

with a soup of pertinent enzymes and fed

molecules called nucleotides, the chem-

ical “letters” of which rnais composed.

Thus supplied, the enzymes use the tem-

plates to run off appropriate mrnas by the

zillion. These are extracted and packaged

into tiny, fatty bubbles to form the vaccine.

Both the cell-culture and the mrna ap-

proaches have benefits and drawbacks.

Vaccines must now be produced on a scale greater than ever before. That is hard

→Alsointhissection

64 Viralvariantsandvaccination

65 Daughtersanddivorce

65 Oasesofplasticinthedeep sea

66 How fish grew legs