COVID-19: Shots alone may not stop surges in larger cities

  • A new study used COVID-19 case data in Chinese cities and anonymized location data from cellphones to model the impact of different levels of mobility and social restrictions.
  • The model predicts that until populations reach “herd immunity” through vaccination programs, measures, such as closures of non-essential businesses and schools, will still be needed in most cities to avoid future stay-at-home orders.
  • However, in cities with low population densities, vaccination programs alone may be enough to prevent resurgences of the virus.

In the summer of 2020, as the number of new COVID-19 cases decreased, many countries in the northern hemisphere relaxed their stay-at-home and physical distancing rules.

Governments wanted to restart their economies and return society to something approaching normality as soon as possible.

However, the relaxation of travel restrictions and distancing rules triggered a second surge of infections in autumn 2020. This, in turn, led to further state-wide and national lockdowns, many of which remain in place today.

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With the rollout of vaccination programs, the urgent question for policymakers is how soon it will be safe to relax restrictions again without risking further lockdowns and their associated adverse effects, both to economies and the well-being of their citizens.

Now, epidemiologists at the University of Southampton’s WorldPop project in the United Kingdom and The Chinese University of Hong Kong in Shatin have created a model using cellphone geolocation data to help plan safe exits from lockdown.

The researchers used the anonymized data to model the effect of different levels of mobility on the number of social contacts people have.

Professor Andy Tatem, director of WorldPop, explains:

“Previous studies assume that when people reduce mobility, they proportionately reduce their social contacts, but this is not necessarily the case. And, as more SARS-CoV-2 vaccines come online, there is an urgent need to understand the relationship between these factors, so we can adjust and tailor interventions and open up sections of society in a safer way.”

The authors define high population density as similar to Wuhan’s, medium as 50% of Wuhan’s density, and low as 30%. Similar high-density cities include Sao Paulo and Bangkok, medium-size cities would include Paris and Rome, while New York, Los Angeles, and San Francisco would constitute low-density cities.

The new model predicts that in cities with low population densities, effective vaccination programs alone will be enough to contain the virus without needing tight restrictions on mobility and social mixing.

In other words, stay-at-home lockdowns will no longer be necessary in these cities.

On the other hand, the authors describe cities with medium or high population densities as having higher natural social contacts and therefore being more at risk of the resurgence of COVID-19 cases as lockdown measures ease.

Their model suggests that effective vaccination programs alone may not be enough to contain the virus without the need for strong measures, such as closing all schools and non-essential businesses and reducing residential interactions by 50%, until cases are under control.

The research appears in the journal Nature Human Behaviour.

Social contact index

The epidemiologists used cellphone geolocation data for people living in Wuhan and four other major Chinese cities to create a “social contact index.”

The index represents a person’s expected daily average number of encounters with other people, given a certain level of mobility.

The geolocation data allowed the researchers to distinguish five types of social contact:

  • residential communities
  • workplaces
  • schools
  • shopping and recreation facilities
  • other facilities

To calculate the effect on transmission that varying degrees of control measures had within different population densities, the researchers incorporated their index into a standard model of how infections spread.

The authors describe three levels of what they call physical distancing, which are more akin to lockdown measures or social restrictions in the United States and Europe:

  • Mild measures: Involve closing schools and a 20% reduction in social contact in the other four categories of social contact.
  • Medium measures: Include closing schools and an 80% reduction in shopping and recreation, such as measures to minimize crowding. These measures could, for example, involve closing parts of facilities or limiting the number of people allowed in a store, bar, or restaurant at any one time.
  • Strong measures: Include the closing of all schools and non-essential business, with a 50% reduction in residential contacts.

The model predicts that as healthcare professionals roll out COVID-19 vaccines, cities with a high population density will still require moderate or strong physical distancing measures. By contrast, cities with low population densities could control the virus with vaccination alone.

Herd immunity

The objective of mass vaccination is eventually to achieve “herd immunity,” when a sufficiently large proportion of a population is immune to an infection to break the chain of transmission.

Under herd immunity, the average number of new infections a person with COVID-19 can generate, known as the basic reproduction rate, or R0, will fall below 1 and stay low. In theory, governments can then lift all social control rules, and the outbreak will still come to an end.

Epidemiologists generally assume that the proportion of the population that must be immune to achieve herd immunity to COVID-19 is around 60%. However, one study suggests that it may be as low as 43%.

There is a snag, however, because people who have a strong immunity to older strains of the virus — either as a result of vaccination or infection with the virus itself — may not be as well protected against newly emerging variants.

However, the authors acknowledge this is a limitation of their model. They submitted their paper for publication in November 2020, before the emergence of several “variants of concern.”

If a new strain can evade the protection afforded by vaccination or previous infection, this could put paid to hopes of achieving herd immunity and easing control measures any time soon.

The model also does not consider other preventive measures, such as wearing face masks and increased ventilation or air purification to reduce spread by aerosol.

In addition, this study used data from China, where people’s behavior in the country’s cities may vary from that in U.S. or European cities. A review of data from U.S. cities showed that while large metropolitan areas were more at risk of high levels of COVID-19 cases, it was connectivity — such as with tourists and business people — rather than density that was most important.

Nevertheless, the finding that in the face of high social interaction, a vaccination rate of around 60% — which experts presume to provide herd immunity — may still not control resurgence alone without further restrictive measures, remains a stark reminder that people still need to take care over the next few months.

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