RIO DE JANEIRO (Reuters) - A long first wave of COVID-19 in Brazil that killed over 150,000 people is subsiding, with public health experts attributing the drop at least in part to a large number of uncounted people who caught the virus and developed temporary immunity.Continued social distancing among those able to work from home, mask wearing, and the warming weather as Brazil heads into the summer months, are also contributing to the fall in new infections, the experts said. But they warn that Latin America’s largest country, which has suffered the second-deadliest outbreak in the world behind the United States, cannot afford to let its guard down. A second wave, such as the one hitting Europe now, remains a possibility, they said. The latest research suggests that post-infection immunity is likely to last at least three months, after which it can wane. Changes in behavior - like people flooding to bars due to a sense the worst is over - can quickly lead to a surge in new infections. Unlike Europe, which managed to contain a first phase of the outbreak relatively quickly by imposing strict lockdowns, Brazil saw its epidemic spiral, with daily deaths plateauing at a high level for months. Brazilian President Jair Bolsonaro has been widely criticized by public health experts for downplaying the gravity of the coronavirus, which he dismissed as a “little flu.” Throughout June, July and August, Brazil recorded about 40,000 new cases and 1,000 deaths daily. Only in September did this trend begin to decline. Last week, deaths averaged 426 per day.