Most people over 80 in Britain will have to wait until next year to get the coronavirus vaccine, a top health official says.
Chris Hopson, chief executive of NHS Providers, said the "vast majority" of people at the top of the priority list won't get a jab until January, February or March.
Professor Stephen Powis, NHS national medical director, said the distribution of the vaccine would be a "marathon not a sprint".
Vaccinations will be administered at dozens of hospital hubs from Tuesday - dubbed "V-Day" by Health Secretary Matt Hancock - with people aged 80 and older, care home workers and NHS workers who are at higher risk at the front of the queue.
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Croydon University Hospital in south London was one of the first hospitals to take delivery of the vaccine over the weekend, with similar scenes unfolding around the country ahead of the rollout.
Mr Hopson said people need to "hang fire" and be assured they have not been forgotten if they have not received a letter or a phone call about the jab.
He said: "I don't think people should expect anything over the next few days because the reality is, as I said, that for the vast, vast, vast majority of people this will be done in January, February, March.
"And the one thing that we don't want people to get anxious about or concerned about is 'Where's my letter?' in December."
He added: "People really shouldn't worry if they're over 80 and they haven't had a letter.
"I'm sure there will be communications over the next few weeks that will tell people how quickly we are getting through the over-80s, and there will be plenty of communications to say, at the right point, if you haven't had a letter then you should talk to your GP, but we are many weeks away from that.
"So as I said people just need to hang fire and wait for a proactive communication.
"If that hasn't happened, don't worry, we haven't forgotten you, and we'll certainly tell you at the point at which you need to start worrying if you haven't been contacted, but that will be many, many weeks away."
The UK has ordered 40 million doses of the Pfizer/BioNTech jab, enough to vaccinate 20 million people, as people need to receive two doses.
There are 800,000 doses in the first tranche, meaning 400,000 people will be vaccinated initially.
There are challenges to overcome with vaccinating care home residents despite them being at the top of the priority list, but Mr Hopson said this would begin in around a week's time and would be led by primary care networks.
Logistical issues mean there are difficulties in delivering the Pfizer jab to residents, as it needs to be stored at minus 70C before being thawed out and can only be moved up to four times within that cold chain before being used.
The vaccine boxes containing 975 doses will need to be split so they can be taken to care homes.
Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA), was asked when the MHRA would approve the breaking up of the vaccine packs into smaller batches for care homes.
She told BBC's Andrew Marr Show: "We have approved how the vaccine can be put into the smaller packs, but obviously what we're doing is giving advice and guidance on how well and carefully that is done."
It has been confirmed care home residents in Scotland will be able to receive the vaccine from December 14.
The distribution of the vaccine across the UK is being undertaken by Public Health England and the NHS in England, Scotland, Wales and Northern Ireland through systems specially adapted from those used for the national immunisation programmes.
On arrival in Croydon, the batch of vaccines was unboxed by a pharmacy technician wearing specific protective equipment to ensure safe handling at such cold temperatures.
After going through final quality control checks, batches will be placed in freezers to ensure they are kept at the right temperature until being used.
There are 50 hubs in the first wave of the vaccination programme in England, with more hospitals starting to vaccinate over the coming weeks and months as the programme ramps up.
It is not known when exactly all 50 hubs will receive vaccine doses, as they are starting to administer the jab at different times, but deliveries are expected throughout the week.
NHS Providers deputy chief executive Saffron Cordery said many hospital hubs had received their allocation of the initial 800,000 doses, and she expected there would be up to four million doses in the country by the end of December.
Meanwhile, the UK's chief medical officers have warned the coronavirus vaccine will only have a "marginal impact" on hospital numbers over the winter.
In a letter to colleagues, the four chief medical officers said this winter would be "especially hard" for the health service due to coronavirus.
GP surgeries in England have also been told to be ready to start staffing GP-led Covid-19 vaccination centres by December 14.
The first to receive the vaccine in these centres will be those aged 80 and over, as long as other risk factors, "clinical or otherwise", have been taken into account.
GPs will then be expected to administer jabs according to the priority list set down by the Joint Committee on Vaccination and Immunisation (JCVI) as more stocks of the vaccine come in.
The British Medical Association (BMA) said there has so far been "mixed messaging" about when higher risk people can expect to be vaccinated.
BMA chairman Dr Chaand Nagpaul said the Government needed to "be crystal clear" about when priority groups will be vaccinated after "mixed messaging about when care homes, high-risk patients in the community and NHS staff can expect to be vaccinated".
