Video report by Lisa Adlam
A 91-year-old man from South Yorkshire is still waiting for his second dose of the Covid vaccine almost 14 weeks after his first jab.
Thomas Sims has been shielding for most of the last year and the delay in receiving his second dose of the Pfizer vaccine has worried his family.
In guidance, the Chief Medical Officers outlined that second doses of both the Pfizer and AstraZeneca vaccinations should be administered up to 12 weeks after the first dose to maximise long term protection.
Research has found protection from any COVID-19 symptoms in the over-80s is 57% from four weeks after vaccination. After the second dose, effectiveness rises to more than 85%.
In the wider population, 72% are less likely to develop infection (with or without symptoms) after one dose of the vaccine, rising to 86% after the second dose.
Dr Carter Singh, a GP, stressed that people should still get a second vaccine even if they have gone over the 12 week period.
He said: "Even if you have gone beyond that timeframe of 12 weeks the most important thing is that you get the second jab when safe and practical to do so because that is going to boost the immune response and give you a longer-lasting immune response against Covid-19."
Doncaster clinical care group said more than 155,000 adults had received their first vaccination and around 26,000 had so far had their second.
In a statement, they added: "We are not aware of any second doses being delayed or of any patient getting in touch to make us aware of a delay.
"If the patient would like to get in touch with the surgery directly, we can look into this further."
Professor Nicola Stonehouse, from the University of Leeds, said that antibody levels do keep going after the first three months and that they do continue after that time.
She added: "We don't know how quickly those antibody levels fall off and decrease, so it is not a case that you are protected for three months and then after three months you're not protected.
"But the important thing is that your level of protection is only at its maximum after you have the second dose and also a few weeks after that second dose, so it is important to get your second dose in a timely manner.
"But, two weeks shouldn't make an enormous amount of difference in the big picture."
She also offered reassurances over the link to blood clots with the AstraZeneca vaccine for under 30s, saying that the decision to offer alternatives to young people was a precaution.