SEOUL, July 26 (Reuters) - South Korea said on Monday it has been informed by Moderna (MRNA.O) of an unspecified production issue involving its COVID-19 vaccine, as the country expands its inoculation campaign for people aged 55-59 amid a fourth wave of infections. It was not immediately clear what the production issue is, how much and long shipment will be impacted and whether other countries will be also affected by the problem. Moderna did not immediately reply to a Reuters" request for comment. "Moderna has informed that it has a production issue," health official Park Jin-young told a briefing, adding that consultations are being held to figure out details. "We"re mobilising all available administrative and diplomatic capabilities to establish facts and devise countermeasures," Park said. South Korea has been struggling to turbocharge its immunisation drive amid supply shortages and shipment delays, with about 33% of its 52 million population having received at least one dose and 13% fully vaccinated. About 6.17 million people, or 84% of those in their 50s, have signed up for this week"s vaccinations, and authorities had already switched to Pfizer (PFE.N)/BioNTech"s (22UAy.DE) products from Moderna for some people in the group because of shipment delays. But the production issue would not affect the total shipment volume planned for this month, Korea Disease Control and Prevention Agency (KDCA) director Jeong Eun-kyeong said, without providing specific numbers. South Korea has a contract for 40 million doses of the Moderna vaccine, of which about 1.1 million have arrived so far. The vaccine rollout for people in their upper 50s had sputtered to a week-long halt this month after a record high number of new cases sparked a rush for shots, exhausting available supplies and crashing an official reservation website. read more . Mass testing and tracing have helped the country suffer lower COVID-19 death rates than other developed countries so far without a full lockdown, but the new wave of infections prompted the government on Sunday to tighten social distancing rules across most of the country this week to head off transmission in the summer holidays. read more The KDCA reported 1,318 new coronavirus cases for Sunday, bringing total infections to 190,166, with 2,077 deaths.