COVID-19: Why herd immunity will likely occur much sooner than currently expected

In a paper sub­mit­ted April 1, 2020, a group of researchers from Italy and Switzerland had esti­mat­ed, via eval­u­a­tion of avail­able data from dif­fer­ent coun­tries and from sta­tis­ti­cal mod­el­ling, that by March 25, 2020, over 10 per­cent of the Italian pop­u­la­tion had already been infect­ed by the nov­el Coronavirus (i.e. more than 6 mil­lion peo­ple), and that the high­est prob­a­bil­i­ty was for the epi­dem­ic to end in Italy “with­in the first week of April or few days lat­er.”¹

The authors based their mod­el­ling sce­nar­ios on the num­ber of deceased, as they right­ly assessed that this be the most reli­able num­ber regard­ing the spread of the epi­dem­ic avail­able at this time. Actual test­ing only cov­ers a small pro­por­tion of the pop­u­la­tion, and is high­ly unre­li­able, for rea­sons we have men­tioned in pass­ing in a pre­vi­ous arti­cle² and which have been cor­rob­o­rat­ed by sig­nif­i­cant anec­do­tal evi­dence we col­lect­ed in our own per­son­al envi­ron­ment, includ­ing among friends and col­leagues in the med­ical com­mu­ni­ty. For their assump­tions regard­ing fur­ther devel­op­ment of the pan­dem­ic and its even­tu­al halt, the authors drew on offi­cial data from China and fit­ted their pro­jec­tion curve to the curve derived from this data.

Independently, a poll con­duct­ed by a lead­ing mar­ket research insti­tute, coor­di­nat­ed by a renowned epi­demi­ol­o­gist from the University of Milan, led to a very sim­i­lar esti­mate for the actu­al infec­tion lev­el. The poll had asked a rep­re­sen­ta­tive sam­ple of the pop­u­la­tion for symp­toms observed on them­selves (and had duly cor­rect­ed, as the authors assure, for the sub­jec­tive nature of the obtained data).³

As would be expect­ed giv­en lim­it­ed test­ing activ­i­ty and even capac­i­ty in most west­ern coun­tries, many oth­er indi­ca­tions have sur­faced for much high­er preva­lence of COVID-19 infec­tions than offi­cial data reveal.⁴

All this implies, of course, that the actu­al death rate from COVID-19 is far low­er than was ini­tial­ly feared, and will even­tu­al­ly set­tle at sig­nif­i­cant­ly less than half a per­cent of those infect­ed. The rea­son for the ini­tial fear of a much high­er lethal­i­ty, one can spec­u­late, is that the Chinese were con­front­ed with a sit­u­a­tion that in real­i­ty was, just as is now the case in Italy, much fur­ther devel­oped than ini­tial­ly esti­mat­ed, so that the Chinese like­ly thought they had a viral dis­ease as lethal as SARS 2003 on their hands, only that it appeared to spread sig­nif­i­cant­ly faster. This ini­tial assess­ment and the dras­tic actions tak­en based on it then shaped much of the pub­lic per­cep­tion of COVID-19 in the West, includ­ing ours.

In the lit­tle over a week since the pub­li­ca­tion of the two men­tioned esti­mates, it has become clear that the esti­mate made by the first-cit­ed group for a plateau­ing of the pan­dem­ic in Italy at around the time of this writ­ing does not con­form to real­i­ty. While the num­ber of dai­ly new deaths has shown signs of declin­ing, the num­ber of total deaths con­tin­ues to rise at a much high­er rate than fore­seen: Instead of an esti­mat­ed total of rough­ly 13,000 deaths by the time of this writ­ing and clear signs of plateau­ing, this num­ber has already sur­passed 18,000.⁵ The esti­mat­ed num­ber would have required the lock­down to yield sig­nif­i­cant results and to halt the pan­dem­ic in Italy before herd immu­ni­ty is achieved. The actu­al num­ber con­forms, in our view, much more close­ly to our own esti­mate, long held and men­tioned in a pre­vi­ous post,⁶ call­ing for effec­tive sat­u­ra­tion around the end of April. By “effec­tive sat­u­ra­tion” we mean herd immu­ni­ty and there­fore essen­tial­ly the end of the pan­dem­ic as regards Italy, not sim­ply a plateau­ing of active case num­bers that would drag out the pan­dem­ic for many more months, with even more dev­as­tat­ing eco­nom­ic con­se­quences, and with many more peo­ple dying “because of” instead of “of” COVID-19.⁷

The actu­al devel­op­ment of the num­bers indicates:

1. Indiscriminate lock­down mea­sures do not work as expect­ed at least in a sig­nif­i­cant part of the west­ern world. Social inter­ac­tion can­not be total­ly pre­vent­ed (peo­ple have to go out to buy food), and where there is inter­ac­tion, there is the risk of infec­tion. For the South Korean solu­tion to rig­or­ous­ly test almost every­one and then iso­late only those test­ed pos­i­tive­ly it is obvi­ous­ly far too late.
2. The phe­nom­e­non of sig­nif­i­cant pre- and pos­si­bly even com­plete­ly asymp­to­matic spread of the virus on the one hand and the unre­li­a­bil­i­ty of test­ing on the oth­er, both due to the observed typ­i­cal course of the infec­tion with ini­tial mas­sive viral repli­ca­tion in the throat that starts declin­ing around symp­tom onset (so that if peo­ple wait – or are put off – for too long to be test­ed, test­ing may yield a false neg­a­tive result), con­tribute to a much faster course of the pan­dem­ic than most have imagined.
3. Therefore, over the com­ing weeks (Italy) to pos­si­bly two months that it should take to effec­tive­ly achieve herd immu­ni­ty in most of the west­ern world, the focus should be put on how best to pro­tect the espe­cial­ly vul­ner­a­ble and how best to man­age those infec­tions that either threat­en to or do indeed get out of hand, in order to max­i­mize those patients’ chances of recov­ery. Calls in recent days for an at least par­tial con­tin­u­a­tion of the lock­down for anoth­er 12 to 18 months, until a vac­cine may (or not) be avail­able, seem, viewed in light of the above, much misplaced.

1. The COVID-19 infec­tion in Italy: a sta­tis­ti­cal study of an abnor­mal­ly severe dis­ease: https://www.medrxiv.org/content/10.1101/2020.03.28.20046243v1.full.pdf[↩]
2. https://loico.com/the-logic-of-surviving-the-coronavirus-pandemic/[↩]
3. Coronavirus, l’indagine: “Almeno 5 mil­ioni di ital­iani infet­tati”: https://www.adnkronos.com/fatti/cronaca/2020/04/02/coronavirus-indagine-almeno-milioni-italiani-infettati_LA4OJM0QbWxViiwQfAsJEO.html[↩]
4. E.g. from mea­sur­ing SARS-CoV-2 titers in waste­water, as has been done in a major urban treat­ment facil­i­ty in Massachusetts: https://www.medrxiv.org/content/10.1101/2020.04.05.20051540v1[↩]
5. https://www.ilsole24ore.com/art/coronavirus-italia-news-ultime-notizie-aggiornamenti-9-aprile-AD2tHIJ[↩]
6. https://loico.com/why-we-have-likely-seen-the-bottom-of-the-corona-crash-in-the-us-stock-market/[↩]
7. https://drmalcolmkendrick.org/[↩]