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<jats:title>Instantaneous contact tracing</jats:title> <jats:p> New analyses indicate that severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2) is more infectious and less virulent than the earlier SARS-CoV-1, which emerged in China in 2002. Unfortunately, the current virus has greater epidemic potential because it is difficult to trace mild or presymptomatic infections. As no treatment is currently available, the only tools that we can currently deploy to stop the epidemic are contact tracing, social distancing, and quarantine, all of which are slow to implement. However imperfect the data, the current global emergency requires more timely interventions. Ferretti <jats:italic>et al.</jats:italic> explored the feasibility of protecting the population (that is, achieving transmission below the basic reproduction number) using isolation coupled with classical contact tracing by questionnaires versus algorithmic instantaneous contact tracing assisted by a mobile phone application. For prevention, the crucial information is understanding the relative contributions of different routes of transmission. A phone app could show how finite resources must be divided between different intervention strategies for the most effective control. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" related-article-type="in-this-issue" xlink:href="10.1126/science.abb6936">eabb6936</jats:related-article> </jats:p>

<jats:p> Van den Heuvel and Tauris argue that if the red giant star in the system 2MASS J05215658+4359220 has a mass of 1 solar mass ( <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub> ), then its unseen companion could be a binary composed of two 0.9 <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub> stars, making a triple system. We contend that the existing data are most consistent with a giant of mass <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mn>3.2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub> , implying a black hole companion of <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mn>3.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.8</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub> . </jats:p>

<jats:p> Thompson <jats:italic>et al</jats:italic> . (Reports, 1 November 2019, p. 637) interpreted the unseen companion of the red giant star 2MASS J05215658+4359220 as most likely a black hole. We argue that if the red giant’s mass is ~1 solar mass, its companion can be a close binary consisting of two main-sequence stars. This would explain why no x-ray emission is detected from the system. </jats:p>

<jats:p>The coronavirus disease 2019 (COVID-19) pandemic has produced the fear and disorder inevitably provoked by emerging pathogens. As such, it should also inspire consideration of our experience with HIV over the past 40 years. As with HIV, the road to reducing infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, the cause of COVID-19), and attendant morbidity and mortality, requires medical and nonmedical strategies. The most important lesson learned from tackling HIV is to use a combination of prevention strategies.</jats:p>