and may also "seed" new strains into an area, instigating community transmission in a pandemic. During the 1918 influenza pandemic, military parades in Philadelphia and Boston may have been responsible for spreading the disease by mixing infected sailors with crowds of civilians. Restricting mass gatherings, in combination with other social distancing interventions, may help reduce transmission. A recent peer-reviewed study in the British Medical Journal (The BMJ) also suggested it as one of the key components of an effective strategy in reducing the burden of COVID-19. Travel restrictions Border restrictions or internal travel restrictions are unlikely to delay an epidemic by more than two to three weeks unless implemented with over 99% coverage. Airport screening was found to be ineffective in preventing viral transmission during the 2003 SARS outbreak in Canada and the U.S. Strict border controls between Austria and the Ottoman Empire, imposed from 1770 until 1871 to prevent persons infected with the bubonic plague from entering Austria, were reportedly effective, as there were no major outbreaks of plague in Austrian territory after they were established, whereas the Ottoman Empire continued to suffer frequent epidemics of plague until the mid-nineteenth century. A Northeastern University study published in March 2020 found that "travel restrictions to and from China only slow down the international spread of COVID-19 [when] combined with efforts to reduce transmission on a community and an individual level. ... Travel restrictions aren't enough unless we couple it with social distancing." The study found that the travel ban in Wuhan delayed the spread of the disease to other parts of mainland China only by three to five days, although it did reduce the spread of international cases by as much as 80 percent. Shielding Shielding measures for individuals include limiting face-to-face contacts, conducting business by phone or online, avoiding public places and reducing unnecessary travel. During the COVID-19 pandemic in the United Kingdom, shielding referred to special advisory measures put in place by the UK Government to protect those at the highest risk of serious illness from the disease. This included those with weakened immune systems (such as organ transplant recipients), as well as those with certain medical conditions such as cystic fibrosis or severe asthma. Until June 1, 2020, those shielding were strongly advised not to leave home for any reason at all, including essential travel, and to maintain a distance from anyone else in their household. Supermarkets