As public health officials continue to urge people across the world to roll up their sleeves for COVID-19 vaccines, coronavirus infections have been on the rise in areas where vaccination rates remain low. That is due in no small part to the delta variant, which is set to become the globally dominant strain in weeks.

The delta variant has taken over in an alarmingly short time. Based on genomic testing, the Centers for Disease Control and Prevention estimates that the variant first identified in India accounted for 10% of cases during the two-week period ending on June 5. Four weeks later, delta's share had swelled to nearly 52%.

And it shows little sign of stopping: Coronavirus infections that had been dramatically reduced in some countries, thanks to COVID-19 vaccinations, are on the rise once again.

Both these facts serve as a testament to the ease with which delta spreads: It is estimated to be 40% to 60% more transmissible than the alpha variant, first identified in the United Kingdom, which itself was about 50% more transmissible than earlier versions of the virus. That means delta spreads more than twice as readily as the coronavirus that initiated the global pandemic.

But what does the delta variant actually do to those it infects? The Times spoke with two infectious disease experts – Dr. Robert Bollinger of Johns Hopkins University and Dr. Otto Yang of UCLA – about what we know so far.