San Juan County, Colorado, can boast that 99.9% of its eligible population has received at least one dose of covid-19 vaccine, putting it in the top 10 counties in the nation, according to data from the Centers for Disease Control and Prevention.
If vaccines were the singular armor against covid’s spread, then on paper, San Juan County, with its 730 or so residents on file, would be one of the most bulletproof places in the nation.
Yet the past few months have shown the complexity of this phase of the pandemic. Even in an extremely vaccinated place, the shots alone aren’t enough because geographic boundaries are porous, vaccine effectiveness may be waning over time and the delta variant is highly contagious. Infectious-disease experts say masks are still necessary to control the spread of the virus.
Airborne transmission by droplets and aerosols is important for the spread of viruses. Face masks are a well-established preventive measure, but their effectiveness for mitigating SARS-CoV-2 transmission is still under debate. We show that variations in mask efficacy can be explained by different regimes of virus abundance and related to population-average infection probability and reproduction number. For SARS-CoV-2, the viral load of infectious individuals can vary by orders of magnitude. We find that most environments and contacts are under conditions of low virus abundance (virus-limited) where surgical masks are effective at preventing virus spread. More advanced masks and other protective equipment are required in potentially virus-rich indoor environments including medical centers and hospitals. Masks are particularly effective in combination with other preventive measures like ventilation and distancing.
Author(s): Yafang Cheng, Nan Ma, Christian Witt, Steffen Rapp, Philipp S. Wild, Meinrat O. Andreae, Ulrich Pöschl, Hang Su