Efficacy of the 3 Shot - Which Covid-19 Vaccine is Better and Booster Side Effects

With the “Delta” or “Indian” variant (B.1.617.2) of the Covid-19 virus still on the prowl in Thailand, accounting for some 90% of infections, understanding the efficacy of the various recommended vaccination regimens is essential. So here goes.

Dealing with the Delta variant

Once they get inside a body, all viruses divide and multiply. The viruses have a tendency to evolve in order to increase their chances of survival and further spread. So the genetic profile of any virus changes with time. In the initial stages of the pandemic we had only Alpha or English, and Beta or South African, variants. But nowadays the most invasive variant is Delta. According to the U.S. Centers for Disease Control and Prevention, Delta spreads as much as 50% faster than Alpha. One person infected with the Delta variant can infect three-four people. That makes an effective and readily available vaccination regimen essential to contain the spread in Thailand, as everywhere else.

Vaccine efficacy testing

To address the issue head-on, the Department of Medical Sciences has joined Siriraj Institute of Clinical Research (SICRES), Siriraj Hospital, to study the level of immunity of vaccinated persons against the Delta variant. Those tests have predominantly focused on vaccines registered and applied in Thailand, namely, Sinovac (SV), AstraZeneca (AZ), Sinopharm (SP) and Pfizer (PZ), to which the team applies the so-called Plaque Reduction Neutralization Test (PRNT). The blood samples are centrifuged to create a serum which is then diluted. In the next stage, the Delta variant of the Covid-19 virus is added. Cell culture medium is then added and the sample is left for a specified period of time. The proportion of plaque or cell groups with at least 50% reduction of inoculation is compared to a control group. This, in turn, gives a Neutralizing Titer 50% (NT50) reading indicating the effectiveness of each formula against infection. The study was conducted on these eight groups of volunteers:

Group 1: Those who received 2 Sinovac (SV+SV)
Group 2: Those who received 2 doses of AstraZeneca (AZ+AZ)
Group 3: Those who received Sinovac, followed by AstraZeneca (SV+AZ)
Group 4: Those who received AstraZeneca, followed by Sinovac (AZ+SV)
Group 5: Those who received 2 doses of Sinovac, followed by Sinopharm (SV+SV+SP)
Group 6: Those who received 2 doses of Sinovac, followed by 1 dose of AstraZeneca (SV+SV+AZ)
Group 7: Those who received 2 doses of Pfizer (PZ+PZ)
Group 8: Those who have been infected

Vaccine efficacy comparative test results against Delta variant

The results showed that two weeks after full dose of vaccination against Delta variant subjects had mean levels of immunity, as measured by Neutralization Titer 50 (NT50), as follows:

Group 1 SV+SV = 24.31
Group 2 AZ+AZ = 76.52
Group 3 SV+AZ = 78.65
Group 4 AZ+SV = 25.83
Group 5 SV+SV+SP = 61.26
Group 6 SV+SV+AZ = 271.71
Group 7 PZ+PZ = 155.07
Group 8 Those who have been infected = 307.29

The results showed that the alternate vaccine (SV+AZ) has higher immunity than two doses of Sinovac (SV+SV) and is equivalent to two doses of AstraZeneca (AZ+AZ). Those who have already had two doses of Sinovac can choose to have Sinopharm or AstraZeneca as a third dose to increase their immunity. However, these findings were based on trials involving only 205 volunteers. As such, they are considered preliminary and further study is ongoing to gain more clarity.
Nevertheless, we hope the findings will be helpful in planning the most effective vaccination course so far. Meanwhile, it’s as important as ever for everyone to do their best to stay healthy and protect those around us by wearing a face mask, regularly cleaning our hands with alcohol spray or gel, and social distancing. When we do this in conjunction with vaccination, it’s the best way to keep ourselves, our loved ones, and everyone, as safe as possible.

• Department of Medical Sciences, Ministry of Public Health
• Corporate Communication, Faculty of Medicine, Ramathibodi Hospital
• Faculty of Medicine, Siriraj Hospital, Mahidol University
• Bumrungrad Hospital