Nov 4 (Reuters) - Moderna Inc (MRNA.O) on Thursday slashed the 2021 sales forecast for its COVID-19 vaccine by as much as $5 billion, grappling to fill vials and distribute them to meet unprecedented world demand, sending its shares tumbling 16%. Moderna executives said production challenges now lie with bottling up doses, also known as fill and finish, and ramping up infrastructure to deliver them internationally, rather than production of raw material. "We really believe we can fix those short-term supply issues .... those are what I would qualify as teething problems of scaling up so fast," Chief Executive Officer Stéphane Bancel told analysts on a conference call. Moderna shares were down 16.3% at $289.50. The U.S. drugmaker is hiring more manufacturing workers and investing additional resources in its production lines to increase the number of doses it can release each week, Bancel said. Moderna"s COVID-19 vaccines, like those of its peer Pfizer, require more demanding cold storage than other shots, creating logistical challenges in shipping them, especially to poorer countries that lack extensive cold storage infrastructure.The company"s sales target cut is in sharp contrast to that of larger rival Pfizer Inc (PFE.N), which earlier this week raised the sales forecast for its shot. read more Moderna"s contract manufacturer Lonza (LONN.S) has been ramping up manufacturing, and Moderna has also enlisted Spain"s Rovi (ROVI.MC), another contract drugmaker that is already bottling Moderna"s vaccine, to build new ingredient production facilities in Granada to lift European output. Bancel said the work was now complete, and "we should see a positive impact from this expansion very soon." Besides production challenges, the vaccine has also been under scrutiny over the rare risk of heart inflammation known as myocarditis. The U.S. Food and Drug Administration (FDA) over the weekend delayed its review of Moderna"s vaccine in children and teens aged 12 to 17 years, to assess the risk of myocarditis. read more Moderna executives said its safety analysis do not show an increased risk in individuals below 18 years of age. "We think, over time, the substantial benefits of our vaccine will ultimately win out here. And so we look forward to continuing to work with the FDA," said Moderna President Stephen Hoge. Moderna said it was now expecting 2021 sales of between $15 billion and $18 billion from $20 billion estimated previously. Deliveries were pegged between 700 million and 800 million doses for the year, down from its prior forecast of between 800 million to 1 billion. Still, Moderna said its sales could be in the range of $17 billion to $22 billion next year.