REFERRING to the article, "Booster for Pfizer, AstraZeneca vaccine recipients now 3 months" (New Straits Times, Dec 28, 2021), previously, only Sinovac recipients were scheduled to receive their booster dose three months after their 2nd dose whereas Pfizer and AstraZeneca recipients had to wait for six months.
The decision to shorten the interval between the 2nd and 3rd (booster) dose for Pfizer and AstraZeneca recipients was driven primarily by the emergence and increasing number of Covid-19 cases attributed to Omicron.
Hence, many Malaysians are now eligible for their booster dose.
While Pfizer remains the recommended choice for booster vaccine by the Health Ministry, Malaysians are given the freedom to choose their preferred booster vaccine.
For example, recipients of two doses of AstraZeneca may contemplate between the Pfizer or AstraZeneca booster. This freedom of choice bestowed by the Health Ministry is commendable as this will allow Malaysians to choose whichever booster vaccine they feel most comfortable with.
For example, recipients of two doses of AstraZeneca may hesitate to switch to a different booster vaccine for personal reasons.
Nevertheless, the process of evaluating and ultimately deciding which vaccine for the booster shot needs to be based on scientific evidence. In the age of misinformation and information overload, this process can be extremely difficult.
For example, we receive piecemeal or contradictory information via WhatsApp and Telegram. Moreover, "expert" opinions may not always be founded on scientific evidence. Lastly, the recent discovery and quick spreading of Omicron further complicates the choice of booster vaccine.
To make an informed decision on whether to receive the booster vaccine and the choice of booster vaccine, we need to consider recent findings reported by the scientific community:
(1) Is the booster vaccine necessary? (2) The safety and immune response to mix-and-matching booster vaccines. (3) The effectiveness of the current vaccines against Omicron.
The archiving aim of this letter is to address these questions based on the latest scientific evidence during a time when Malaysians are gearing up for their booster vaccination.
Some vaccines such as smallpox and polio last a lifetime. Unfortunately, Covid-19 vaccines do not. Real-world data on vaccine effectiveness in the United Kingdom during the Delta-dominant period demonstrated Pfizer effectiveness was 92 per cent, 86 per cent, and 78 per cent after 30, 60, and 90 days from the 2nd dose.
AstraZeneca effectiveness was 70 per cent, 67 per cent, and 63 per cent after 30, 60, and 90 days from the 2nd dose. Real-word data on vaccine effectiveness in Israel also demonstrated increased risk of infection in adults 90 days after receiving their 2nd dose of Pfizer compared to adults less than 90 days after receiving their 2nd dose.
The evidence indicates that protection conferred by Covid-19 vaccines wanes with time and a 3rd dose is necessary.
The COV-BOOST clinical trial studied the safety and immune response using seven different booster vaccines following two doses of either Pfizer or AstraZeneca.
For recipients of Pfizer, the booster vaccine that conferred the highest level of neutralising antibodies against Delta was Moderna, followed by Pfizer and AstraZeneca. Similarly for recipients of AstraZeneca, the booster vaccine that conferred the highest level of neutralising antibodies against Delta was Moderna, followed by Pfizer and AstraZeneca.
It is noteworthy that overall neutralising antibody levels following booster dose were higher for previous recipients of Pfizer compared AstraZeneca. Serious adverse events were rare, similar rate in participants receiving one of the seven Covid-19 vaccines as the booster and participants receiving control (non-Covid-19) vaccine as the booster.
Before the emergence of Omicron, Delta constituted over 99 per cent of reported Covid-19 cases. The basic reproduction number (R0) for the Delta variant is 7 whereas the current estimates of R0 for Omicron is as high as 10.
With a higher R0, Omicron is predicted to supersede Delta as the dominant strain (just as Delta superseded the original virus strain and other variants).
Indeed, in countries with high number of cases such as UK, Martin Hibberd, professor of emerging infectious diseases at London School of Hygiene & Tropical Medicine, predicts Omicron to overtake Delta as the dominant variant in a matter of time.
A very recent study published in the Lancet on Dec20 demonstrated minimal antibody response to Omicron in individuals vaccinated with two doses of AstraZeneca. However, there were some antibody response to Omicron in individuals vaccinated with two doses of Pfizer.
Nevertheless, the antibody response to Omicron was lower compared to Delta or the original virus strain. At time of writing, there were no peer-reviewed studies on the booster dose against Omicron.
Taken together, the protection conferred by current Covid-19 vaccines wane with time, as soon as three months after the 2nd dose, hence a 3rd (booster) dose is necessary to ensure continued herd immunity.
Current Covid-19 vaccines as boosters following two doses of either Pfizer or AstraZeneca elicit immune response against Delta.
Finally, studies available on the degree of protection conferred by the booster dose against Omicron remain limited and we should continue to monitor the scientific literature on this pertinent topic to guide national vaccination policies.
Sean Wen is a Member, Malaysian Centre for Genomic Analytics and DPhil candidate in Medical Sciences, University of Oxford