Su­per­spread­ers key to COVID-19 trans­mis­sion

The Charlotte Observer (Sunday) - - News - BY SHASHANK BEN­GALI

In the bleak rank­ing of worst COVID-19 out­breaks, the United States, with 7.3 mil­lion in­fec­tions, is likely to be eclipsed only by In­dia, which has 6.5 mil­lion cases but is catch­ing up fast.

Yet parts of In­dia have led the world in one as­pect of the pan­demic re­sponse: con­tact trac­ing – the la­bor-in­ten­sive, time­sen­si­tive, painstak­ing work of iden­ti­fy­ing peo­ple who were ex­posed to a known in­fected per­son.

Ex­ten­sive con­tact trac­ing in two south­ern In­dian states of­fers the strong­est ev­i­dence yet that a few su­per­spread­ing in­di­vid­u­als are re­spon­si­ble for a dis­pro­por­tion­ate share of new coro­n­avirus in­fec­tions, ac­cord­ing to a study pub­lished Wed­nes­day in the jour­nal Sci­ence. It also sug­gests that chil­dren are more ef­fi­cient trans­mit­ters of the virus than widely be­lieved.

A team of In­dian and U.S. re­searchers ex­am­ined data from 575,071 in­di­vid­u­als who were tested af­ter com­ing into con­tact with 84,965 peo­ple with con­firmed cases of COVID-19. That’s an av­er­age of seven con­tacts per case, and a co­hort more than 10 times larger than in a pre­vi­ous study from South Korea that mapped how the virus was trans­mit­ted.

“It’s the largest epi­demi­o­log­i­cal study any­where on COVID by far,” said the lead au­thor, Ra­manan Laxmi­narayan of the Cen­ter for Disease Dy­nam­ics, Eco­nom­ics and Pol­icy, in New Delhi.

Laxmi­narayan and his col­leagues found that just 8% of peo­ple with COVID-19 ac­counted for 60% of the new in­fec­tions ob­served among the con­tacts. Mean­while, 7 out of 10 COVID-19 pa­tients were not linked to any new cases.

The find­ing un­der­scores the es­sen­tial role of su­per­spread­ers in the COVID-19 pan­demic: One in­di­vid­ual or event, such as in a poorly ven­ti­lated in­door space, can trig­ger a high num­ber of new in­fec­tions, while oth­ers might not trans­mit the virus at all.

In the new study, re­searchers tracked down 78 peo­ple who had shared a bus or train with one of eight known in­fected peo­ple and sat within three rows of that per­son for more than six hours. Health work­ers vis­ited th­ese con­tacts at their homes to con­duct fol­lowup screen­ings and de­ter­mined that nearly 80% of them had con­tracted the coro­n­avirus.

By con­trast, peo­ple who were known to be ex­posed to in­fected in­di­vid­u­als in lower-risk en­vi­ron­ments – such as be­ing in the same room but more than 3 feet away – be­came in­fected only 1.6% of the time.

“Su­per­spread­ing events are the rule rather than the ex­cep­tion,” Laxmi­narayan said. “It has lots of im­pli­ca­tions for mod­el­ing COVID, for how to keep places safe.”

The study sug­gests that su­per­spread­ing events are in­flu­enced by be­hav­ior – that prox­im­ity to an in­fected per­son, length of con­tact and am­bi­ent con­di­tions de­ter­mine the level of risk. It doesn’t ex­am­ine whether some in­fected peo­ple spread the virus more ef­fi­ciently be­cause of bi­o­log­i­cal fac­tors, a ques­tion sci­en­tists are still try­ing to an­swer.

The results could help guide safety mea­sures in places such as gyms, churches and choir prac­tice spa­ces that have been lo­ca­tions for pre­vi­ous su­per­spread­ing events.

The study also found that al­though chil­dren younger than 17 were the least likely to die of COVID-19, they trans­mit­ted the virus at rates sim­i­lar to the rest of the pop­u­la­tion, un­der­scor­ing the idea that the disease doesn’t spare young peo­ple. One data point in par­tic­u­lar holds im­pli­ca­tions for re­open­ing schools: Chil­dren ages 5 to 17 passed the virus to 18% of close con­tacts their own age.

An­to­nio Salas, a Span­ish researcher who has in­ves­ti­gated the role of su­per­spread­ers in the pan­demic, said the study’s find­ings re­gard­ing chil­dren were im­por­tant in light of “pre­vi­ous re­ports sug­gest­ing a mi­nor role of chil­dren in the pan­demic.”

“Na­tional poli­cies on how to pro­ceed with chil­dren in schools and other so­cial ac­tiv­i­ties could change dra­mat­i­cally if the sci­en­tific ev­i­dence un­der­pins the idea that chil­dren can in­fect as ef­fi­ciently as adults, and even more, they could also be­have as su­per­spread­ers,” said Salas, who was not in­volved in the In­dia study.

As In­dia’s coro­n­avirus caseload has dou­bled over the past month, from 3 mil­lion to more than 6 mil­lion, the study au­thors said their work showed one strength of the coun­try’s re­sponse: the abil­ity to mo­bi­lize large num­bers of health work­ers and civil ser­vants to con­duct con­tact trac­ing, iden­tify high­risk in­di­vid­u­als and closely track their cases.

The two In­dian states in the study, Andhra Pradesh and Tamil Nadu, have a com­bined pop­u­la­tion of 128 mil­lion and boast some of the largest health care work­forces and high­est lev­els of pub­lic health spend­ing in the coun­try. Both turned to disease-sur­veil­lance net­works put in place years ago during the AIDS epi­demic to make house-to-house checks at lev­els unseen else­where in the world.

In Tamil Nadu, which in­cludes the coastal metropo­lis of Chen­nai (for­merly known as Madras), thou­sands of pub­lic and pri­vate health care work­ers were trained as con­tact trac­ers and health sur­veil­lance of­fi­cers. The state had ex­pe­ri­ence iden­ti­fy­ing at-risk pop­u­la­tions from its bat­tle against AIDS, which struck Tamil Nadu harder than al­most any­where else in the coun­try.

Once a per­son tested pos­i­tive for the coro­n­avirus, a pub­lic health worker in­ter­viewed them by phone about where they’d been and whom they’d met over the pre­vi­ous two weeks. Im­me­di­ate fam­ily mem­bers were tested and iso­lated right away; oth­ers were lo­cated through phone calls and text mes­sages.

Nearly all the con­tacts traced were peo­ple known to the in­fected per­son, so many strangers were missed. Still, in Chen­nai, health work­ers lo­cated and tested an av­er­age of 17 con­tacts per in­fected per­son through Aug. 1.

Al­though con­tact trac­ing is cru­cial to iden­ti­fy­ing, iso­lat­ing and testing those vul­ner­a­ble to in­fec­tion – and helped coun­tries such as South Korea avoid dis­as­trous out­breaks – an ef­fec­tive sys­tem has eluded most of the U.S. be­cause of short­ages of trained staff and fund­ing.


A med­i­cal worker col­lects a swab sam­ple from a man for a Rapid Anti­gen Test for the COVID-19 coro­n­avirus, in Sri­na­gar, In­dia, on Thurs­day.

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