As everyone knows now, vaccines are key to fighting the spread of viruses, with Covid-19 being an alarming case in point. But as they naturally endeavor to spread more effectively, viruses mutate. Vaccine development must therefore continue in order to counter new waves that can spread more rapidly and cause epidemic severity to increase, as it has been doing in many countries. Besides working on new vaccines, researchers around the globe have also been studying how combining different vaccines, or ‘cross-vaccination’, can possibly increase efficacy, especially against new variants such as Delta.
How vaccination impacts the immune system
To get a clearer understanding of cross-vaccination efficacy, we need to know more about how the immune system responds to vaccination. A vaccine with or without antigens is basically a way of stimulating the body to produce antibodies against the virus in question. This takes up to 30 days or sometimes longer, but once it occurs, the body is able to remember the antigen and make quickly make antibodies as required to counter any potential infection.
Vaccines that contain spike protein genes such as AstraZeneca, and those that contain mRNA genes such as Pfizer, instruct the body to produce the antibodies necessary to prevent, in this case Covid-19 virus, infecting cells.
Meanwhile, the body’s natural immune system, comprising T-Cells, joins forces with the vaccine antibodies to destroy the targeted foreign substance before it can spread and do damage. Basically, antibodies marked to communicate with T-Cells that the virus is a foreign body, attach themselves to the surface of the cells. So triggered, the T-Cells destroy the infecting virus.
AstraZeneca-Pfizer cross-vaccination efficacy study results
Cross-vaccination became part of Thailand’s vaccination program the moment the Pfizer vaccine was added in. Understandably, people are wondering how its addition affects overall efficacy. So far, the only available data comes from Com-COV, a Covid-19 vaccination testing entity of the University of Oxford in England.
Com-COV studied what happens when two doses of vaccine in various combinations are administered. Using 830 random subjects with a mean age of 57.8 years, 45.8% female and 54.2% male, the trial divided four groups according to their vaccinations: Group 1 AZ+AZ, Group 2 PZ+AZ, Group 3 AZ+PZ, Group 4 PZ+PZ. Here’s what they found:
• Group 1 AZ+AZ – antibody level: 1,392 ELU/ml. T-Cell response level: 50 SFC/106
• Group 2 PZ+AZ – antibody level: 7,133 ELU/ml. T-Cell response level: 99 SFC/106
• Group 3 AZ+PZ – antibody level: 12,906 ELU/ml. T-Cell response level: 185 SFC/106
• Group IV PZ+PZ: antibody level: 14,080 ELU/ml. T-Cell response level: 80 SFC/106
Group 4, with both doses of PZ, had the highest antibody levels but a moderate T-Cell response which pays an important role in building long-term immunity. Conversely, Group 3 with AZ followed by PZ had the second-highest antibody levels coupled with very high T-cell response.
The university still recommends having two doses of the same vaccine, at least until more studies of cross-vaccination are done, and is now looking at what happens when you introduce the Moderna vaccine into the mix.
• Department of Mental Health
• University of Oxford Research Paper
• BBC News