Mix-and-match COVID vaccines trigger potent immune response

A news article in Nature describes a study led by the Carlos III Health Institute in Madrid that explored the benefits of combining different coronavirus vaccines. Preliminary results show that vaccinating people with both the Oxford–AstraZeneca and Pfizer–BioNTech COVID-19 vaccines produces a potent immune response against the virus SARS-CoV-2. The trial of more than 600 people — announced in an online presentation on 18 May — is the first to show the benefits of combining different coronavirus vaccines.

Because of safety concerns, several European countries are already recommending that some or all people who were given a first dose of the Oxford–AstraZeneca vaccine get another vaccine for their second dose. Researchers hope that such mix-and-match COVID-19 vaccination regimens will trigger stronger, more robust immune responses than will two doses of a single vaccine, while simplifying immunization efforts for countries facing fluctuating supplies of the various vaccines.

Arterial events, venous thromboembolism, thrombocytopenia, and bleeding after vaccination with Oxford-AstraZeneca in Denmark and Norway

A recent article in the BMJ describes a population based cohort study that assessed rates of cardiovascular and haemostatic events in the first 28 days after vaccination with the Oxford-AstraZeneca vaccine ChAdOx1-S in Denmark and Norway, and compared them with rates observed in the general populations.

The researchers observed increased rates of venous thromboembolic events, including cerebral venous thrombosis, among recipients of ChAdOx1-S. For the remaining safety outcomes, the researchers observed slightly higher rates of thrombocytopenia/coagulation disorders and bleeding, which could be influenced by increased surveillance of vaccine recipients. The absolute risks of venous thromboembolic events were, however, small, and the findings should be interpreted in the light of the proven beneficial effects of the vaccine, the context of any given country, and the limitations to the generalisability of the study findings.

The critical role of background rates of possible adverse events in the assessment of COVID-19 vaccine safety

An article in the March edition of Vaccine explains why it is necessary to know the background rates of possible Adverse Events of Special Interest (AESI) when monitoring the safety of COVID-19 vaccines. The article provides examples to illustrate the use of background rates and discusses tools available from the Brighton Collaboration that facilitate case evaluation and understanding of AESI.

The UK Expert Haematology Panel (EHP) has produced guidance on the syndrome of Thrombosis and Thrombocytopenia occurring after coronavirus vaccination

The Expert Haematology Panel is comprised of UK blood clotting experts, who are meeting daily to collect information on possible cases and are in regular communication with the regulators and other haematology groups studying this potential problem in other European countries. Their guidance includes a link to a living document and the contact details for advice from the Expert Haematology Panel. They state that it is important that correct management is applied to prevent the progression of thrombosis. Of critical note, platelet transfusions should be avoided.

Why is it so hard to investigate the rare side effects of COVID vaccines?

A news item in Nature by Ariana Remmel explains how challenging it is to prove that a medical problem following immunization was caused by the vaccine itself and describes the kinds of studies that are needed to understand the causes of rare adverse events. These include targeted clinical trials and active surveillance systems that collect adverse event data, both background rates and rates after vaccination, from electronic health records.