The slow rollout of coronavirus vaccines around the world is already causing governments to adjust their plans.
Joe Biden's incoming administration in the United States plans to follow the move by some European countries to concentrate on getting first doses out to as many people as possible, as quickly as possible.
That way, more people will get some protection, although two doses are required for longer-lasting immunity. Key vaccines approved so far - made by Pfizer/BioNTech, Moderna and AstraZeneca/Oxford - require second shots with scheduled waits of three to four weeks between jabs.
The Trump Administration has been holding back millions of shots, while ensuring people can get a second shot.
President-elect Joe Biden is counting on companies to produce second doses on the scheduled time. Reportedly, he will make greater use of the Defence Production Act, which allows the government to step up manufacturing during emergencies, if necessary.
Britain, up to 12 weeks, and Denmark, up to six weeks, are widening the dose gap. Germany and Ireland are investigating extensions to the wait time. British Columbia in Canada is stretching it to 35 days.
Adapted approaches run the risk of fuelling public scepticism towards authorities and resistance to vaccines in some quarters.
So far the US is well behind on its targets, which has major implications for the international economic recovery and safe international travel.
About 6.6 million people have received their first shot, and about 22 million doses have been sent to states. The American Hospital Association says 1.8 million people need to be vaccinated daily until the end of May to reach widespread immunity by mid-year. The US is running more than a million people per day below that.
State and local public health departments say there has been a lack of support and funding. A member of the Biden transition's coronavirus advisory board, Dr Celine Gounder, told CNN the problem is one of distribution, not supply, and that it is key to simplify the process.
According to Ourworldindata.org, Israel has been the most effective country in running a Covid-19 vaccination programme so far with a rate of 20.6 jabs administered per 100 people in the population. The United Arab Emirates has the next best rate with 11.
Countries which started quickly in December - the US, United Kingdom and Canada - are in the top 10. Most are smaller nations in terms of population or size: Israel, the UAE, Bahrain, Denmark, Italy, Slovenia and Lithuania.
That suggests New Zealand, which also has extra time to plan, has a good chance of running a smooth vaccination programme - once doses land.
Australia's chief medical officer Professor Paul Kelly says that most Australians will get the AstraZeneca/Oxford vaccine rather than the Pfizer/BioNTech shot. Both are also coming here. The Oxford shot is easier to handle since it can be stored in normal fridges.
Supplies of the AstraZeneca/Oxford vaccine are being produced for Australia in Melbourne whereas the other vaccine is being sourced overseas and supplies are limited, Kelly said.
Australia has secured 10 million doses of the Pfizer vaccine and 53.8 million of the AstraZeneca vaccine. Vaccinations are expected to begin next month.
Countries in Asia apart from China have been slow to get vaccinations under way but China and India, with their production capacity, will be crucial to vaccines getting to people in developing countries.
The World Bank warned this month that any delay to rollouts risks slashing the growth rate for 2021 and a "slow and challenging recovery" is the best outlook.