Case control studies are a staple in the epidemiologist’s toolbox. The basic idea idea is that instead of carrying out an analysis on a random sample of a population, you instead sample from the population conditional on occurrence of an outcome of interest. Typically this will involve selecting all individuals who have the outcome (the cases), and only some people who don’t have the outcome (the controls). The motivation for doing this is that when the outcome is rare, most of the precision in estimates is driven by the cases, and adding a large number of controls often has little effect. Case control studies can thus be more efficient in terms of data collection/processing compared to e.g. a cohort study.

In November 2021, I was invited by Health Data Research UK (HDRUK) to give a talk as part of a public webinar on vaccine safety. One of the goals of this webinar was to allow members of the public to ask questions and interact with professionals carrying out research in this area.

Myself and a team of collaborators including Stuart Bedston at the University of Swansea, Declan Bradley at Queen’s University Belfast and Mark Joy at the University of Oxford recently published a paper in the International Journal of Epidemiology studying waning of COVID-19 vaccine effectiveness. One of the major findings of the paper is that for first and second dose of AstraZeneca and first dose of Pfizer, effectiveness of the vaccine against COVID-19 hospitalisation or death reached zero by approximately day 60-80 after receiving the vaccine, and then went negative.

On February 22nd, PLOS medicine published a paper on cerebral venous sinus thrombosis (CVST) following vaccination on which I was the lead author. CVST is a type of blood clot in the brain that can lead to stroke and death. In this paper, we carried out a pooled analyis of 11.6 million people in England, Scotland and Wales, comparing the rate of CVST events in individuals before and after receiving the vaccine. We found that in the 4 weeks following vaccination with Oxford-AstraZeneca, the risk of CVST events approximately doubled. We did not see any increase in risk of CVST events following Pfizer.

A paper has recently come out claiming that lockdowns had minimal effects on COVID mortality, causing quite a stir. The headline figure is that in Europe and the USA, lockdowns reduced mortality by 0.2%. Fifty words in and the authors are already dropping bombs like this:

There have been interesting reports from Public Health Scotland that COVID infection rates amongst the unvaccinated have been consistently lower compared to those who have had one or two doses of vaccine since early December 2021, though higher compared to those with a booster dose. See a news story here, based on the original report here. I believe this has led many people to question vaccine effectiveness.

There has been a great deal of controversy over the way COVID-19 deaths are calculated. Initially in the UK, the count that was most widely reported consisted of all who died within 60 days of a positive PCR test, or who died more than 60 days after a positive test but had COVID-19 listed as a cause of death on their death certificate.

The main objection to this is that, for example, someone could test positive for COVID-19, recover fully, then die in e.g. a car crash and still be counted as a COVID-19 death.