A fourth surge of Covid-19 is filling hospitals around the United States, fueled by the relaxing of mitigation measures, the more transmissible Delta variant and lagging vaccination rates. In states and counties where Delta is surging, contact tracers are overwhelmed. But monitoring the spread of the virus remains vital to getting it under control. The public health response to a Covid-19 outbreak in Provincetown, Mass., in July offers an object lesson in how Delta changes the playing field and what’s needed to manage the pandemic’s next phase. Given the opportunity, the coronavirus (and the Delta variant in particular) loves spreading in clusters, meaning that lots of people get infected over a short period, sometimes at the same time and by the same person. In early July, Provincetown was not short on opportunities. Thousands of people were driven indoors by rain and did not wear masks, mixing and matching in nightclubs, bars and restaurants. Even though most revelers were vaccinated, it’s no surprise that multiple superspreading events occurred. What happened next is instructive. Noticing an uptick in cases, investigators in Massachusetts tried to identify clusters of transmission and where they happened. Where you find one transmission event, you are likely to find more. This approach is called “source investigation,” or “backward contact tracing.” When a case is reported, investigators look back at a person’s activities to identify where they might have been infected, and then determine whether others may have been infected at the same time. The technique has proved useful for analyzing disease clusters in countries like Australia and New Zealand, where every case of unknown origin is considered an emergency because it could mean that a potential superspreading event has not been found. In the United States, outbreaks are easy to identify in congregate settings like prisons and nursing homes. With schools back in session and Covid-19 cases emerging on campuses, school districts can narrow down a list of people potentially exposed to the coronavirus by reviewing classroom rosters and attendance. But contact tracing protocols in many states are not well suited for superspreader outbreaks outside of those controlled settings, because they often focus on who the case might have infected, not on the location where the infection occurred. Patrons at restaurants and bars are largely anonymous; each person typically knows only a handful of others. Source investigation complements standard contact tracing; it’s not a substitute. When outbreaks are detected, people need to be informed immediately, so they can get tested and ensure they are not unknowingly spreading the virus. Once a cluster is detected, venues should re-evaluate their Covid mitigation measures and ensure that staff are vaccinated and ventilation systems refresh indoor air at least four to six times per hour. High-risk venues should keep lists of patrons, and outreach to customers through social media can be very effective. Mandating that customers are vaccinated, as New York, San Francisco and New Orleans have done, may not suffice to prevent outbreaks. Some European countries are requiring evidence of a negative coronavirus test for unvaccinated people entering indoor entertainment venues. The Provincetown outbreak occurred as thousands of people arrived to celebrate the Fourth of July, believing their vaccinations would protect them against infection. When infected people began feeling ill, some didn’t immediately attribute their symptoms to Covid-19 because of the widespread belief that breakthrough infections were almost impossible. After the cases emerged, the local health department’s investigation was undoubtedly helped by strong word-of-mouth communication, as many infections affected members of the gay community, a group that for decades has witnessed the effects of the AIDS pandemic. Since then, many local businesses have reassessed their Covid mitigation measures, the town reinstituted an indoor mask mandate and new cases have leveled off. The Provincetown investigation also made quick, impressive use of genetic sequencing. This permitted officials to identify the Delta variant as the culprit behind the outbreak, and, as The Times reported in July, it helped the Centers for Disease Control and Prevention recognize that vaccinated people could be infected and transmit the coronavirus to others. This led to revised guidance that suggested that vaccinated people should wear masks indoors where there is high community transmission. The tools and techniques that the Massachusetts Department of Public Health used in Provincetown are not special and should be used much more widely. All over the country, large groups are gathering, often in poorly ventilated spaces. Nightclubs and bars are packed in many cities, and most have much lower vaccination rates than Provincetown. Just because we are not looking for outbreaks in these locations does not mean that they are not happening. Are Florida’s rocketing hospitalizations being driven by myriad little Provincetowns? Are contact tracers looking? Some officials have expressed concern that reporting on breakthrough cases diminishes the public’s confidence in vaccines. Unfortunately, the C.D.C. is not tracking mild breakthrough cases. We disagree with this decision. Unless states are reporting standardized data about breakthrough cases, it will be difficult for the C.D.C. to determine how much they are contributing to community transmission. For the moment, vaccinated people should be encouraged to take common-sense precautions like getting tested and staying home if they develop symptoms of Covid-19. Provincetown shows how source investigation can shed a powerful light on how the virus spreads anonymously in the dark, among crowds, amid dancing and music. While vaccination is the single most effective tool we have to fight Delta, it does not reduce transmission to zero. So we must also employ other weapons in our arsenal: source investigation, masks indoors when cases are surging and readily available testing. Our pandemic future may remain unwritten, but fatalism in the face of Delta will only lead to yet more unnecessary illness and death. The New York Times