The Economist - USA (2021-12-18)

48 International TheEconomistDecember18th 2021

proteins  on  the  virus  particles’  surface.

That  is  why  vaccines  can  still  protect

against disease even if the antibodies they

provoke no longer recognise the pathogen

as well as they did originally, or if they have

waned over time.

That  said,  a  better  antibody  response

would be nice to have; slowing the rate of

infections  would  slow  the  spread  of  dis­

ease  and  give  health  systems  breathing

space. This is where vaccine booster shots

come in. Boosters improve all forms of im­

munity: one of their effects is to raise anti­

body  levels,  at  least  for  a  while.  This  in­

creased quantity can go some way to mak­

ing  up  for  the  reduced  quality  of  their  re­

sponse,  lowering  the  risk  of  infection.

Boosters  may  improve  the  quality  of  the

antibodies, too; the more the immune sys­

tem  sees  a  virus,  the  better  attuned  to  it

some antibodies become. 

A reasonable expectation that vaccines

offer  protection  against  serious  disease,

especially  after  a  third  jab,  is  one  piece  of

good news. Another may be that Omicron

infection  leads  to  less  severe  disease  all

round. There is some evidence of this from

Gauteng, the South African province where

the variant has run rife. Data from Discov­

ery  Health  suggest  adults  with  Omicron

have  a  29%  lower  hospital  admission  risk

relative  to  that  seen  in  the  country’s  first

wave of covid­19 in mid­2020. The propor­

tion  of  those  hospitalised  who  end  up  in

intensive care is much lower than in previ­

ous waves, too, and fewer of those on gen­

eral wards need supplemental oxygen. An­

gelique Coetzee of the South African Medi­

cal Association, who was one of the first to

raise the alarm about Omicron, has consis­

tently argued that it is a milder variant. 

A paper recently submitted for peer re­

view  by  Michael  Chan  and  colleagues  at

Hong Kong University suggests one reason

why this might be. They found that in the

first few days of infection Omicron repro­

duced 70 times more readily than Delta in

the airways leading to the lungs. But in the

lungs  themselves  it  reproduced  ten  times

less well than earlier variants. 

The details of how viruses cause disease

depend  on  a  lot  more  than  simple  repro­

ductive  rates.  But  this  finding  might  go

some way to explaining a lower incidence

of  severe  disease;  it  is  infection  in  the

lungs that does most damage. And greater

replication  higher  up  the  respiratory  tract

might improve transmissibility. Being very

good  at  getting  into,  and  reproducing  in,

the lining of the airways could make it eas­

ier for the virus to set up shop in someone

exposed to it. What is more, a lot of activity

in the airways might also mean more parti­

cles  get  back  out  into  the  air.  Indications

that  the  symptoms  of  Omicron  infection

are  more  like  those  of  the  common  cold

might fit with this interpretation.

But neither that laboratory work nor the

datafromSouthAfricaamounttoa strong

casethatOmicronwillbea lotlessdanger­

ousthanearlierstrainseverywhere.The

SouthAfricandataarepreliminary;sofar

theycoveronlythefirstthreeweeksafter

infection.Typically,newwaves ofcovid

variantsstartinyoungergroupsandwork

theirwayintoolder,morevulnerablepop­

ulationsovertime.Andtheratesatwhich

infectionleadstoseverediseaseanddeath

candifferbetweencountriesandpopula­

tions.Factorsatplaybeyondtheyouthof

theSouthAfricansmightincludethefact

thatmostvaccinationsinthecountryare

relatively recentandthefactthatafair

number have been previously infected.

Theremay also besalient factorsstem­

mingfromgeneticvariationorpriorhealth

histories.Thingscouldlookquitedifferent

in older populations elsewhere in the

worldwhichhaveseenfewerinfections.

Dacapo,moltovivace

Thedegreetowhichthevariantcaninfect

the previously infected may also make

thingslookrosierthantheyreallyare.Nat­

alieDean,a biostatisticianatEmoryUni­

versity,pointsoutthat Omicron’ssuccess

atreinfectingpeoplemaygivetheimpres­

siona smallerfractiongetsseverelyilljust

byinflatingthedenominator.Itcouldthus

seemmorebenignevenif,amongthose

contractingcovidforthefirsttime,itwere

justasdangerousasDelta(seechart).

Asthedebateaboutthecomparativese­

verity of the infectiongoes on,public­

health officials are stressing that what

matterstotheindividualandtothehealth

systemarenotallthatwellaligned.Foran

individual,a lessdeadlyvariantisprefera­

bletoa moredeadlyone,regardlessofhow

transmissibleitmaybe.Fora healthsys­

tem,thenumberofcasesatanygiventime

isa criticalconcern,whichmakestherate

oftransmissioncruciallyimportant.There

isa levelbeyondwhichthesystemcannot

copewiththenumberofhospitalisations.

Afast­spreadingviruscanreachthatlevel

evenifitproducesa lowerproportionof

severecasessimplybecausethetotalnum­

berofcasesatanygiventimeissohigh.

Toprovidea senseofthis,researchersat

theLondonSchoolofHygieneandTropical

Medicinehavecomparedmodelsforthe

spreadofOmicronwiththesituationin

Englandduringtheworstpreviouspeak,in

early2021.Oftheirvariousscenarios,the

onewhichcurrentlylooksmostplausible

makes Omicronprettygoodatinfecting

peoplewhohavebeenvaccinatedorinfect­

edbutalsotreatsboostersasbeingquite

goodatstoppingit.Thatwouldproducea

peakinhospitaladmissionsinlateJanuary

welloverthe3,800a dayseenin2021.It

would lead to 23m­30m infections be­

tween now andMay 2022, and 37,000­

53,000deaths.Modelsfromthesameteam

have,inthepast,provedoverlygloomy,but

theybelievetheyunderstandwhyandhave

madeappropriateadjustments.

The prospectof hospitalisation rates

thathighsawEnglandgointotwonational

lockdowns, one in November 2020 and

oneinJanuary2021.Thistimethegovern­

ment has recommended working from

homeandreintroducedsomeinfection­

controlmeasures,suchasmask­wearing

on public transport. It may introduce

more.Itsgreateststress,though,isona

hell­for­leather dash to provide booster

shotstoalladultsbyJanuary1st.

Ina reportpublishedonDecember15th

theEuropeanCentreforDiseasePreven­

tionandControlwarnedthata rangeofen­

hancedprecautionswerenownecessary,

including reducedcontact insocial and

work settings, fewer large gatherings,

moremask­wearingandmoretesting.Re­

ducingtravelandmixingbetweenhouse­

holdsandgenerations overtheholidays

mayalsobeonthecards.Somecountries

are sure to seetoughermeasures soon;

someindividualsarealreadytakingthem.

Butif theperceptionthatOmicronisnotso

dangerous—whether well founded or

not—becomeswidespread,peoplemaysee

littlereasontoadheretostricterrules.

CountrieswhereOmicronratesarestill

verylowhavea littlemoretimetoprepare,

tolearnfromthosefurtheralongthecurve,

andtoestimatewhatisnecessarytoflatten

andlowerthepeak.Butthegrowthrates

seensofarstronglysuggestthattimeis

bestmeasuredindays,maybeweeks.Ex­

ponentialgrowthisa dizzyingthing.n

Selection eects

Covid-19 infections by severity and re-infection

capacity, illustrative example

Sourc: NatalieDean,EmoryUniversity,Atlanta

Severe Severe

Not

Severe

Symptoms

Susceptible

population

Previously

infected

population

Infection

averted

Moderate

Mild

Asymptomatic

Variant with little capacity for re-infection

Severe

Not

Severe

Severe

Symptoms

Infection

averted

Re-infected

Moderate

Mild

Asymptomatic

Susceptible

population

Previously

infected

population

Variant with high capacity for re-infection