Vaccinate potential superspreaders first, researchers say

With the Pfizer/BioNTech vaccine reporting interim results of 90% efficacy, it looks as though the world is coming closer to obtaining a vaccine to wield against SARS-CoV-2.

This leads to the question of how best to use the vaccine resources we have to achieve the best outcome, particularly as supplies may be initially limited. To help address this question, Macquarie University researchers have unveiled a strategy that models who should be vaccinated first, publishing their findings in PLOS ONE.

A theoretical model developed by researchers at Macquarie University, QUT and CSIRO has laid the groundwork for the new vaccination strategy, which aims to have the biggest impact on suppressing the spread of the coronavirus.

The strategy — which identifies locations visited by people who are most likely to become superspreaders and targets them for vaccination — may assist policymakers in their plans to roll out a vaccine, once it has been proven to be safe and effective at the completion of clinical trials.

Tracking superspreaders

The research team used anonymised location data for the movements of 600,000 people in Shanghai and Beijing who were on a messaging app called Momo. The team analysed a staggering 56 million location visits in just 71 days.

“Focusing on the locations where a potential superspreader visits, and vaccinating all direct and indirect contacts in the cluster at those locations, is the most effective method,” said Professor Bernard Mans from Macquarie’s Department of Computing.

“We found this approach would be as good as vaccinating identified superspreaders based on an accurate contact list, and significantly better than random vaccinations.”

The researchers calculated all the people that the superspreaders would have come in direct and indirect contact with, then extrapolated these trends to develop a model to test the theoretical effectiveness of a vaccine strategy.

Using the location data, people were ranked into six classes by the number of places they’d visited — the higher the number, the more contacts. For example, Class 1 meant the person had only stayed at home or visited local shops and been in contact with up to five people. A person in Class 2 would have also gone to a coffee shop or a bus stop and been in direct and indirect contact with up to 15 people.

A Class 3 person would have gone to a local train station or the office, a small park or a swimming pool and been in close proximity to up to 25 people, whereas a Class 6 person had visited highly populated public places like universities, airports and stadiums and potentially been in direct and indirect contact with more than a hundred people. The Class 6 people are clearly more likely to become superspreaders.

“Our research shows that to be effective (and realistic), it’s not about who exactly a superspreader has been in contact with 24/7, but where they’ve been, that should be the focus of vaccination,” Professor Mans explained.

He added that past research has shown that people often won’t reveal all their contacts for privacy reasons or they forget all the people they’ve met.

Professor Mans said that, until now, current theoretical vaccination strategies have not been designed to capture indirect transmissions and potentially miss out on individuals’ numerous indirect links, such as people who get into a lift after them. Focusing on location means you can easily discover indirect as well as direct contacts.

Initially, when the researchers began their analysis, they were creating predictive models to determine who should be vaccinated against influenza or any disease with potential indirect transmission. With the onset of COVID-19, they expanded their scope to focus on vaccines for a global pandemic.

The team have also assessed strategies for a preventative and a reactive approach. In the case of COVID-19, because the pandemic has already spread across the world, it is obviously too late to prevent it.

If this vaccine strategy is to be put into practice once a COVID-19 vaccine is ready, Professor Mans emphasised the importance for public locations such as restaurants, cafes, clubs, shops, and sporting and entertainment facilities to generate an accurate list of patrons so that they could be traced and vaccinated.

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