The Oxford Covid-19 vaccine is better tolerated in older people compared to younger adults, and produces a similar immune response in old and young adults.

Early results from a phase 2 trial, published in The Lancet, show similar safety and immunogenicity results in healthy older adults (aged 56 years and over) to those seen in adults aged 18-55 years.

It also found that the vaccine causes few side effects, and induces immune responses in both parts of the immune system in all age groups and at low and standard dose – provoking a T cell response within 14 days of the first dose of vaccination, and an antibody response within 28 days of the booster dose of vaccination.

Phase 3 trials are ongoing to confirm these results – as well as how effective the vaccine is in protecting against infection with SARS-CoV-2 – in a broader range of people, including older adults with underlying health conditions.

Study lead author Professor Andrew Pollard, University of Oxford, UK, says: “Immune responses from vaccines are often lessened in older adults because the immune system gradually deteriorates with age, which also leaves older adults more susceptible to infections. As a result, it is crucial that Covid-19 vaccines are tested in this group who are also a priority group for immunisation.”

Phase 2 trial results of Oxford Covid-19 vaccine

In the trial, 560 participants (160 aged 18-55 years, 160 aged 56-69 years, and 240 aged 70 or over) were split into 10 groups where they received either the ChAdOx1 nCoV-19 vaccine at a low or standard dose, or a control vaccine (the meningococcal conjugate vaccine). Participants aged over 55 years were also split into groups and either given a single dose of vaccine, or two doses 28 days apart.

Study recruitment occurred during a national lockdown in the UK when vulnerable individuals were advised to self-isolate. For this reason, the study includes only healthy participants and not those with comorbidities or who are frail. Before receiving the vaccine, all participants had a blood test to determine if they had previously been infected with SARS-CoV-2. Those who had antibodies for SARS-CoV-2 were excluded, except for 18-55-year-olds in the standard dose double vaccine groups.

Following vaccination, participants were observed for a minimum of 15 minutes in case of any immediate adverse events, and participants recorded any adverse events for seven days afterwards. Participants will continue to be monitored for any serious adverse events for one year following final vaccination (the year long data are not yet available).

Participants aged 18-55 years who received two standard doses of the Oxford Covid-19 vaccine and all participants aged 56 years or over had their immune responses assessed on the day of vaccination, then 1, 2 and 4 weeks after their first and second vaccination.

Adverse reactions to the ChAdOx1 nCoV-19 vaccine were mild (the most common effects were injection-site pain and tenderness, fatigue, headache, feverishness and muscle pain), but more common than seen with the control vaccine. Thirteen serious adverse events occurred in the six months since the first dose was given, none of which were related to either study vaccine.

New study answers some questions about protecting older adults

Co-author, Professor Sarah Gilbert, University of Oxford, UK, says: “The WHO has outlined a number of critical factors for Covid-19 vaccines, including that they must be targeted at the most at-risk groups including older adults. They must also be safe, effective in preventing disease and/or transmission, and provide at least six months of protection for people frequently exposed to the virus – such as healthcare workers.

"Our new study answers some of these questions about protecting older adults, but questions remain about effectiveness and length of protection, and we need to confirm our results in older adults with underlying conditions to ensure that our vaccine protects those most at risk of severe Covid-19 disease.” 

The authors note some limitations to their study, including that the participants in the oldest age group had an average age of 73-74 years and few underlying health conditions, so may not be representative of the general older population, including those living in residential care settings or over 80 years of age. Larger studies are now underway to evaluate immunogenicity, safety and efficacy in older adults with a wider range of comorbidities.

Lastly, the authors note that almost all participants of all ages were white and non-smokers, and may not be representative of the general population, but people from a range of backgrounds, countries and ethnicities are being included in the phase 3 trial of this vaccine.

Writing in a linked Comment, lead author Dr Melissa Andrew, Dalhousie University, Canada, who was not involved in the study, says: “It is encouraging that more studies in older adult populations are underway and will hopefully bring opportunities to implement nuanced analyses of how underlying health status and frailty affect vaccine safety, reactogenicity, immunogenicity, and efficacy in older adults in real-world settings. Older adults (across the full spectrum of frailty) and those who care about them are eagerly awaiting this progress towards safe and effective Covid-19 vaccines.”