“The policy is that we certainly want everyone who gets a first dose to get their second dose when you’re dealing with a two-dose regimen,” Anthony Fauci said on Monday. He noted, though, that “we don't want to hold back” doses. ”If a person is ready for their second dose, that person will be prioritized. If a person has not gotten their first dose, but you’ve taken care of all of the second-dose people, then you go ahead with the first dose.” As hospitalizations and deaths from Covid-19 rise, governments are grappling with whether to prioritize getting a first shot of vaccine into as many arms as possible, or to take a slower approach that reserves some of the limited supply to deliver a second dose at the optimal interval to attain maximum protection. Some governments seem willing to accept the trade-off of lower immunity to inoculate more people. Some scientists have cautioned that untested timing regimens have the potential to undermine confidence in vaccines and, even worse, facilitate the emergence of dangerous new variants. What’s the timing on vaccine doses? Covid-19 vaccines made by Moderna, Pfizer.-BioNTech, and AstraZeneca are administered in two doses. The recommended timing of the second dose varies depending on the vaccine, and is based on clinical trial data regulators used in their review process. In studies, the second Pfizer-BioNTech dose was given 21 days after the initial shot; Moderna’s was given 28 days later. Doses of AstraZeneca’s vaccine were scheduled 4 to 12 weeks apart across four trials. Why is a second shot needed? Vaccines are intended to hardwire immunological memory to generate a rapid, targeted and durable response by antibodies and T-cells to an invading foe. Typically booster shots are intended to provide the immune system with advanced “training” to make better antibodies. Some vaccines requiring multiple doses are administered four months apart to optimize immune memory. Covid-19 vaccines may work the same way, but it will take additional studies to know this for sure. What’s being proposed? By using shots that normally would have gone to people who have already received the first dose, the U.K. aims to give more people an initial dose. That followed a move by U.S. health authorities to revise guidance that recommended an interval of three or four weeks between doses of the Moderna and Pfizer-BioNTech vaccines. The new guidance said doses can be spaced out by up to six weeks if it’s impossible to get the follow-up shot on time. The FDA said slight delays shouldn’t affect the protection offered by the vaccine. What will the delay do? It’s hard to know without more research. Data from the Pfizer-BioNTech trial found partial protection was achieved as early as 12 days after the first dose, but two doses were required to provide maximum efficacy, or 95% protection against disease. There are no data to demonstrate that protection after the first dose is sustained after 21 days. There is also concern that inoculated patients may change their behavior in a way that increases the risk of spread, negating some of the benefits of vaccination. In a split with other regulators, the European Medicines Agency said that changing how coronavirus vaccines are given to people raises the risk of reducing effectiveness. What are the risks of spacing out doses? They’re medical, theoretical and social. The medical risk is that the first dose almost certainly won’t be as protective as two doses. Spacing out doses extends the period when people will have incomplete protection until they get their second dose. The theoretical risk is that a weak response to the virus in millions of people who have received just one dose would foster the emergence of new variants that could evade vaccine-induced antibodies. This would undermine a year’s work to develop safe and effective vaccines. And the social risk is that mixed messages on the topic — from governments, regulators, professional groups and drugmakers — will make it more difficult to communicate with the public on vaccines that are already controversial, and shunned by some. Subscribe to our YouTube channel: https://bit.ly/2TwO8Gm Bloomberg Quicktake brings you live global news and original shows spanning business, technology, politics and culture. Make sense of the stories changing your business and your world. To watch complete coverage on Bloomberg Quicktake 24/7, visit http://www.bloomberg.com/qt/live, or watch on Apple TV, Roku, Samsung Smart TV, Fire TV and Android TV on the Bloomberg app. Have a story to tell? Fill out this survey for a chance to have it featured on Bloomberg Quicktake: https://cor.us/surveys/27AF30 Connect with us on… YouTube: https://www.youtube.com/user/Bloomberg Breaking News on YouTube: https://www.youtube.com/c/BloombergQuickTakeNews Twitter: https://twitter.com/quicktake Facebook: https://www.facebook.com/quicktake Instagram: https://www.instagram.com/quicktake