By Mary Hearty

As the world continues to feel the evolution of variants, COVID-19 vaccine booster shots are likely to contribute to community immunity. This is because they help increase the levels of antibodies in the fight against the coronavirus hence preventing more infections and transmission.

Associate Professor Helen Petousis-Harris, a vaccinologist, and the Director of the Vaccine Datalink and Research Group at the University of Auckland, who also co-leads the Global Vaccine Data Network, made these remarks during a special global media briefing dubbed: Getting your head around the vaccine data, organized in collaboration with the Africa Science Media Centre (AfriSMC) and science media centres in Australia, New Zealand, UK, USA, Germany, Japan, Israel, Taiwan, Philippines, Thailand, Singapore, Austria, France, Indonesia and Hong Kong.

She stated: “Booster shots might prevent more infections and transmission because in order to prevent more infections, we really need more of these antibodies. They could help improve immunity to variants and also help people who have not responded well to the first dose.”

The vaccinologist cautioned that the more the hosts the more the opportunity for variants to arise, and some might be more effective at evading vaccine immunity, especially in Africa where majority of the population are yet to receive the vaccines. Nevertheless, with booster shots, they might contribute to herd immunity, she said.

In terms of how the vaccines work, she explained: “Normally, the first exposure to a vaccine initiates a primary immune response which matures over time. So even if the antibody levels decline, other aspects of the immunity are busy refining themselves.

“Then during the secondary immune response when another dose of vaccine is given, a massive expansion of the immunity is observed as the antibody levels increase. Therefore, in order to have a secondary immune response, there need to be an interval between the two doses,” Dr. Petousis-Harris.

She noted that antibodies have been observed to wane however, the immune response continues to mature overtime, and breakthrough infections have been increasingly observed in people vaccinated months ago. Besides, most declines have been observed among older people.

Professor Kim Mulholland, a Professorial Fellow at the University of Melbourne Department of Paediatrics, and Group Leader at the Murdoch Children’s Research Institute, Melbourne said: “A study conducted in the laboratory to test the neutralizing ability of the serum of people who have received Pfizer and AstraZeneca vaccines suggests that two doses are needed for the variants.”

According to Prof Mulholland, the measurement of vaccine effectiveness is not straight-forward; most COVID-19 vaccines are effective, but they are less effective in preventing mild disease and virus circulation.

Fortunately, their effectiveness probably wanes over time. For instance, he said that Israel has been highly successful with vaccination although this alone is unlikely to control the Delta variant.

Prof Neil Ferguson, an Epidemiologist, and Director of the MRC Centre for Global Infectious Disease Analysis, Imperial College London advised that there should be more studies on vaccine effectiveness as variants like the Delta variant which is more transmissible could escape the effectiveness of the vaccines to some extent.

In the case of vaccine safety, Dr. Petousis-Harris suggested that while all authorized vaccines had favourable safety profiles in the clinical trials, larger numbers of vaccines are needed to exclude the risk of rare and very rare events. “To exclude the risk of rare events, you need a larger population like 4 million,” she said.

“We assessed vaccine safety using many approaches such as passive surveillance, enhanced surveillance, rapid cycle safety monitoring, phase IV and post-marketing studies, and then the clinical trials.”

Passive surveillance is whereby both the public and health professional can voluntarily report events that happen after vaccination. “This enables us to pick up events that might not be in the earlier trial, also known as early warning signs,” Dr. Petousis-Harris said.

With regards to enhanced surveillance, she elaborated that this involves asking people how they are after the vaccine. On the other hand, rapid cycle safety monitoring and phase IV post-marketing studies involves comparing millions of vaccinated and un-vaccinated people and the trial is coordinated in about 17 countries across the globe.

Following vaccine safety assessment, the serious events causally associated with COVID-19 vaccines are blood clots, Guillain Barre Syndrome (GBS) in viral vector vaccines and myocarditis in mRNA vaccines.