The Covid-19 pandemic now appears to be the worst it has ever been, with daily new cases worldwide topping 800,000 several times over the past week. More new cases have been reported in the past two weeks than in the first six months of the crisis.

More than one in three of these new cases were reported in India, the world’s second-most-populous country and now the epicenter of Covid-19. The rising infections, deaths, and strained health system have created a humanitarian crisis, one that may not relent for months.

Other parts of the world that have barely begun to vaccinate people may soon see their own surges in Covid-19.

What’s complicating the situation is the rise of new variants of SARS-CoV-2, the virus that causes Covid-19. Several of these variants contain mutations that can make prior immunity less effective, allow the virus to spread more readily, and, in some cases, cause more deaths. And the more the disease spreads, the more variants can arise.