The next Covid-19 variant that will rise to world attention will be more contagious than omicron, but the real question scientists need to answer is whether or not it will be more deadly, World Health Organization officials said Tuesday.

Roughly 21 million Covid cases were reported to the WHO over the last week, setting a new global record for weekly cases from the rapidly spreading omicron variant, Maria Van Kerkhove, WHO’s Covid-19 technical lead, said during a livestreamed Q&A across the group’s social media channels.

While omicron appears to be less virulent than previous strains of the virus, the sheer volume of cases is crushing hospital systems worldwide.

“The next variant of concern will be more fit, and what we mean by that is it will be more transmissible because it will have to overtake what is currently circulating,” Van Kerkhove said. “The big question is whether or not future variants will be more or less severe.”

She warned against buying into theories that the virus will continue to mutate into milder strains that make people less sick than earlier variants.

“There is no guarantee of that. We hope that that is the case, but there is no guarantee of that and we can’t bank on it,” she said, noting that people should heed public safety measures in the meantime. What’s more, the next iteration of Covid may also evade vaccine protections even more, making the existing vaccines even less effective.

Pfizer and BioNTech on Tuesday started testing a Covid vaccine that specifically targets the omicron variant, as concerns grow that the current shots aren’t holding up against infections and mild illness caused by the strain discovered just over two months ago.

The U.S. Centers for Disease Control and Prevention found in a study published last week that a booster dose of Pfizer’s vaccine was 90% effective at preventing hospitalization from omicron 14 days after the third shot was administered.

Booster doses are also up to 75% effective at preventing symptomatic infection from omicron two to four weeks after the third shot, according to data from the U.K. Health Security Agency published earlier this month. However, the study found that boosters weaken substantially after about 10 weeks, providing 45% to 50% protection against symptomatic infection.

While omicron appears to have peaked in some countries, it’s gaining ground in others, WHO officials said. “You won’t have to wear a mask forever and you won’t have to physically distance, but for now, we need to keep doing this,” Van Kerkhove said.

The virus will continue to evolve before it settles into a pattern, said Dr. Mike Ryan, WHO’s director of emergency programs. He said it will hopefully settle into a low level of transmission with potentially occasional epidemics. It could become more seasonal or may only affect vulnerable groups, he said.

The problem, he said, is that Covid is unpredictable.

“The virus has proven to give us some nasty surprises,” Ryan said. World health officials need to continue tracking Covid as it evolves, he said, and be ready “if there’s a nasty surprise that we can at least put in place measures again that will stop this new variant doing any more damage.”