Moscow health officials launched an ambitious antibody screening program Friday to evaluate the population’s immunity levels as coronavirus cases continue to grow across Russia, with more than 260,00 cases reported nationwide.

In a blog post explaining how the effort will work, Moscow Mayor Sergei Sobyanin said that every several days 70,000 residents will be selected and invited into one of 30 state clinics across the city to be given an enzyme test for Immunoglobulin M antibodies. Sobyanin said the effort was necessary to provide an informed answer to the question of when Moscow can begin lifting a strict lockdown that has been in place for the past six weeks, and is currently not scheduled to end before May 31.

Sobyanin’s decision would seem to be a subtle rebuke to President Vladimir Putin, who earlier this week lifted a federal stay at home order and declared Russia was now ready to begin a gradual easing of restrictions — despite growing case numbers that surpassed a quarter of a million by mid-week, propelling Russia’s outbreak to be the second largest in the world.