Africa will likely see increasing numbers of coronavirus cases in coming weeks because it’s “very likely” that some cases are slipping through the net, the head of the Africa Centre for Disease Control said on Thursday.

“We are picking some people but we are also missing some people,” said Africa CDC director John Nkengasong. “The situation will get worse before it gets better because the chances are clear that people have slipped through.”

The virus has multiplied in Africa more slowly than Asia or Europe, but 34 African nations have now reported a total of more than 600 coronavirus cases. Worldwide, it has infected nearly 220,000 people and killed nearly 9,000, according to a Reuters tally.

Nkengasong said the number of confirmed cases in Africa was expected to rise in coming days and travel bans would delay but ultimately fail to contain the virus.

“Anyone who has followed pandemics over the years you know that that doesn’t work,” he told a news conference. “When you lock down countries, you should understand clearly how to unlock the country.”

One possible scenario, he said, was “people are coming in and they are slipping through and we are not picking them. It is very likely.”

Nkengasong said testing was going to increase as more kits became available. U.S. company Abbott, Swiss-based Roche Diagnostics and California-based Cepheid’s GeneXpert were all ramping up production, he said. The testing could be rolled out quickly through existing infrastructure for HIV testing, he said.

“We have now three companies that are now producing commercially available tests,” he said. “We have received 2000 tests from China and we have since passed that on to supporting countries. We are expecting to receive another 10,000 tests from China, hopefully next week.”

Many countries worldwide have suspended flights, closed their borders and banned public gatherings to curb its spread.