Prokaryotic CRISPR-Cas adaptive immune systems insert spacers derived from viruses and other parasitic DNA elements into CRISPR loci to provide sequence-specific immunity1,2. This frequently results in high within-population spacer diversity3–6, but it is unclear if and why this is important. Here we show that, as a result of this spacer diversity, viruses can no longer evolve to overcome CRISPR-Cas by point mutation, which results in rapid virus extinction. This effect arises from synergy between spacer diversity and the high specificity of infection, which greatly increases overall population resistance. We propose that the resulting short-lived nature of CRISPR-dependent bacteria–virus coevolution has provided strong selection for the evolution of sophisticated virus-encoded anti-CRISPR mechanisms7.

Background Climate change threatens to undermine the past 50 years of gains in public health. In response, the National Health Service (NHS) in England has been working since 2008 to quantify and reduce its carbon footprint. This Article presents the latest update to its greenhouse gas accounting, identifying interventions for mitigation efforts and describing an approach applicable to other health systems across the world. Methods A hybrid model was used to quantify emissions within Scopes 1, 2, and 3 of the Greenhouse Gas Protocol, as well as patient and visitor travel emissions, from 1990 to 2019. This approach complements the broad coverage of top-down economic modelling with the high accuracy of bottom-up data wherever available. Available data were backcasted or forecasted to cover all years. To enable the identification of measures to reduce carbon emissions, results were disaggregated by organisation type. Findings In 2019, the health service's emissions totalled 25 megatonnes of carbon dioxide equivalent, a reduction of 26% since 1990, and a decrease of 64% in the emissions per inpatient finished admission episode. Of the 2019 footprint, 62% came from the supply chain, 24% from the direct delivery of care, 10% from staff commute and patient and visitor travel, and 4% from private health and care services commissioned by the NHS. Interpretation This work represents the longest and most comprehensive accounting of national health-care emissions globally, and underscores the importance of incorporating bottom-up data to improve the accuracy of top-down modelling and enabling detailed monitoring of progress as health systems act to reduce emissions. Funding Wellcome Trust.

While vastly differing in scale, each nation's health sector directly and indirectly releases greenhouse gases (GHG) through energy consumption, transport, and product manufacture, use, and disposal. Until recently, however, the health sector has not been measured as a coherent segment of the world's climate footprint. A 2019 report by Health Care Without Harm, in collaboration with Arup, provides the most comprehensive global analysis of health care's contribution to climate change to date, based on full global coverage of spending data together with detailed information from 43 countries. It identifies key sources of health care emissions while allowing for comparison between nations and among many regions of the world. Among the report's key findings are that health care's climate footprint is equivalent to 4.4% of global net emissions; that the top three health care emitters-the United States, China, and the European Union-comprise more than half of the global footprint; and that 71% of emissions are derived from the health care supply chain. The findings inform a series of international, national, and subnational policy recommendations for health care climate action, and it identifies opportunities for further research and methodological development to support the health sector in its efforts to understand and address its climate footprint. Additional findings from 2020 will also be presented. Ultimately, these estimates of health care's climate footprint provide the baseline information needed to devise pathways and track progress toward health care de-carbonization across the sector.