Why did the covid-19 epidemics mostly go away in China, northern Italy and the New York City area but then ignite in other places around the world?

One explanation is that the virus ran out of new people to infect in those early sites. In any community, there are those with active infection, those previously infected and immune from reinfection, those immune because of a prior, related coronavirus infection and those susceptible. The basis of the concept of “herd immunity” is if enough people in a community are immune due to prior infection or vaccination then the likelihood an infected person spreads the virus to a susceptible person is too low to maintain transmission.

But how do we apply that understanding to China, Italy and New York, where the epidemic declined but nowhere near enough people got infected to reach the commonly accepted threshold of herd immunity?

One possible answer is people do not interact at random, and thus the infection does not spread equally within the larger community. We have seen that in most of the covid-19 epidemics, some places and population groups get infected at a much higher rate than others. Within those heavily impacted groups, there is much greater person-to-person contact or exposure than within the population overall. Once infection — and subsequent immunity — rates are high enough in that those subgroups, there is a low chance of infecting someone, and the infection disappears.

That non-random contact pattern is consistent with what epidemiologist call “core theory.” Core theory has been around since the 1970s and was first used to explain how certain sexually transmitted infections (STIs) could still circulate even when most people only have one or two sex partners. If person A has two partners and becomes infected by partner 1, and the STI is only 50 percent contagious, then person A has a 50 percent chance of passing the infection to partner 2. Over time, if this pattern is repeated, STI spread should slow and eventually burn out. That is not what we see, however, and STIs continue to cause problems and disease.

That’s because a small proportion of people, what is called a core group, have many more than one or two sex partners. In any given year, that core group may have five, or 10 or more partners, and they maintain the spread of STIs within a smaller group of people — who also have a larger number of sex partners. Occasionally, someone from the larger population interacts with someone from the core group and gets infected, but they rarely spread it to others. A core group is like a fire burning in the center of a building that heats the whole structure. If we can control or put out the fire, the entire building cools off.

The same thing is happening with the spread of covid-19. There are core groups of individuals who — because of their occupation or living situation — have much higher levels of close person-to-person contact than the general population. Examples might include factory workers inside poorly ventilated settings, people living in crowded households and those incarcerated or living in long-term-care settings.

The beauty of the theory is that once the groups who have high levels of close contact reach herd immunity, either through infection or vaccination, the virus’s ability to maintain an epidemic in the larger population collapses, and the fire goes out. Which means the bottom line for us is that we do not need to achieve a population-level immunity of 60 percent to 70 percent. We only need to reach immunity or, better yet, epidemic control within core groups to see the virus decline.

The best evidence of how core theory may apply to the covid-19 pandemic is the recent data from Corona, Queens, where 68 percent of people tested had antibodies to the virus, suggesting recent infection, or in certain towns in Italy where 50 percent to 60 percent of residents had evidence of recent infection. In both cases, the regional or country-level positivity in those areas is much lower: In New York City, it’s 10 percent to 15 percent, and in Italy, it’s 5 percent. But both regional epidemics declined because the core groups were no longer fueling further infection in the larger population.

The implications of such an understanding of how the epidemic spreads and maintains are profound. What it tells us is that we must target our resources — public health communication, requirements for mask-wearing, regular screening for symptoms, testing for asymptomatic infection, paid sick leave policy, social distancing and hygiene — to core groups, like those living in crowded households or working in dense indoor settings, and to people who move between core groups and those most vulnerable, like adult day-care workers or nursing home staff. We do not need to close all businesses, eateries, schools and beaches, but we do need to keep those most at risk protected by controlling the infection in core groups. We need to provide paid furloughs to those over 65 years of age and keep them out of classrooms, and remind younger people to stay away from the elderly without masks or social distancing.

And we must have better and more available data on exposures and populations, so we know exactly where to target interventions. It also means that when a vaccine becomes available core groups need to be at the front of the line. The better we use our understanding of how and where the virus is spread, the sooner we can bring the virus under control.

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