How long does COVID vaccine immunity last?

The results from a huge clinical study into COVID vaccine immunity have just been published. The study, one of the largest ever conducted in the UK, tracked over 6 million people’s vaccination status and matched it with COVID infection.

The results contain illuminating data on just how effective the vaccinations used in the UK are at preventing hospitalisation and death from COVID. It shows how long protection from death and serious illness is proven to last.

Why was the study done?

Since vaccines against COVID-19 infection began to be used in the UK in December 2020, many studies have shown that vaccines are effective at preventing severe disease and death. However, the effectiveness of the vaccines wanes over time, yet by how much has not been certain.

In one recent large study in Qatar, which tracked just under 1 million participants, no evidence of protection against infection was seen at 20 weeks (or beyond), following mRNA vaccination.

Being sure of the speed at which the immunity wanes is critically important to ensure public health, the right timing for any boosters and planning any future vaccination campaigns.

What was tracked?

The study tracked outcomes from vaccination with AstraZeneca, Pfizer and Moderna vaccines, in order to investigate the waning of protection against infection, related hospitalisation and death.

A total of 7,106,982 people who had PCR tests via NHS Test and Trace were assessed. Of these, 6,056,673 (85%) were successfully linked to National Immunisation Management System (NIMS) database. NIMS records which vaccines were administered to patients and when they were given. That allowed the researchers to calculate the time between vaccination and hospitalisation or death.

Those who were vaccinated and had PCR confirmed positive infection were compared with a control group. The control group was those who had been vaccinated, had been PCR tested, yet tested negative. The control group had 3,763,690 participants.

Vaccine effectiveness was then assessed for each vaccine, according to the intervals after vaccination (at least 28 days after the first dose and at least 14 days after the second dose).

The effectiveness of the AstraZeneca and Pfizer vaccines was assessed according to age and co-existing conditions over time, since receipt of the second vaccine dose. That in turn allowed the researchers to investigate waning of effectiveness separately for the Alpha and Delta variants (the study was conducted before the arrival of the Omicron variant).

What were the results?

2,376,037 participants (39.2%) received two doses of the AstraZeneca vaccine and 2,133,769 (35.2%) received two doses of the Pfizer vaccine. Some 176,235 (2.9%) had received two doses of the Moderna vaccine, and 12,169 (0.2%) had received a mixed course or an interval of less than 19 days between doses – this last group was excluded from the study. The remainder were unvaccinated (22.5%).

Immune data has indicated that antibody levels can wane relatively rapidly after the receipt of two doses of vaccine, although decreases in antibody levels may happen faster than a reduction in protection.

The researchers observed waning in vaccine effectiveness against hospitalisation and death at 20 weeks or more after vaccination, with both the AstraZeneca and Pfizer vaccines.

The study found some evidence that protection against symptomatic infection started to drop from 10 weeks after the second dose of the AstraZeneca and Pfizer vaccines. The Moderna vaccine maintained very high levels of protection against hospitalisation at 10 weeks post-vaccination (100% effectiveness).

Protection against hospitalisation and death was kept at high levels for at least 20 weeks after receipt of the second dose for all the vaccines.

Waning of vaccine effectiveness was greater in older adults (those over 65) and in those in a clinical risk group.

Who ran the study? Can the results be relied upon?

The study was organised and led by the UK Health Security Agency. It included some of the most respected medical bodies in the country, with regard to infectious diseases.

The National Institute of Health Research (NIHR), London School of Hygiene and Tropical Medicine Imperial College London, Guy’s and St. Thomas’ Hospital NHS Trust, St. George’s University of London and the University of Oxford were all involved in the study.

The authors of the study themselves stated that because the study design was observational, the risk of potential bias cannot be excluded.

The estimates of vaccine effectiveness relate to those who sought testing and were successfully matched to the NIMS database, so they may not be representative of the whole UK population. For example, a higher proportion of non-White persons than White persons did not match to the NIMS database.

The study also relied on people declaring their symptoms when a PCR test was requested. It is possible that some asymptomatic persons may have declared symptoms in order to obtain a test, however, the numbers are not likely to have affected the results.

How long will vaccine protection last?

The study found that the reduction in vaccine efficacy against symptomatic disease was consistent with other large studies in Israel and Qatar. The results showed that only limited waning of protection against hospitalisation and death was consistent with other studies.

The message from tracking over 6 million vaccinated people is clear: the vaccines all give very good protection against becoming infected or sick from COVID, for up to 10 weeks. The vaccines’ ability to prevent infection and illness can wane from 10 weeks, yet they are proven to be highly effective at preventing hospitalisation and death for at least 20 weeks.

Although the study proves the effects of vaccination on sickness, hospitalisation and death up to 20 weeks post-vaccination, it is possible the life-saving effects may last for much longer. Only time (and more large studies) will tell.

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