The whole idea of ramping up the vaccination rate is to achieve herd immunity, whereby a large enough critical mass is immune against Covid-19, thereby conferring indirect protection to the unimmunised cohort of the population.
Herd immunity is a function of the Reproductive Number RO. Herd Immunity (HI) = 1-(1/RO).
If the RO of the virus is accepted at the low of 6.0 (ranges between 6.0 – 8.0), then the herd immunity would be equal to 1 – (1/6) = 0.83.
83 per cent of the population needs to be immunised in order to achieve population immunity.
With 16 per cent of those surveyed refusing to be immunised and another 17 per cent unsure about the Covid-19 vaccine, it is highly unlikely that herd immunity will be achieved, and protection of the population will not be feasible with vaccination alone.
It would seem that the waves of Covid-19 would only dwindle or cease when virtually everyone has achieved immunity, either through vaccination or infection by the virus.
Then, the SARS-CoV-2 will become an endemic coronavirus, like its four other predecessors that cause the common cold.
89 per cent of the more than 100 experts surveyed by the scientific magazine Nature responded that SARS-CoV-2 was either very likely or likely to become an endemic virus.
Herd immunity also requires the candidate vaccine to be effective against all forms of Covid-19 infection, asymptomatic and symptomatic, not just the more severe forms.
From the outset, the vaccines were designed to prevent the severe manifestations of Covid-19, namely hospitalisations, ICU admissions and deaths.
It has this done very well, even in the face of the more transmissible and virulent Delta variant. In fact, it has surpassed the vaccine efficacy of the biannual influenza vaccination, which pivots at about 60 per cent.
We will therefore need a high efficacy vaccine to achieve a herd immunity of 80 per cent. The vaccine efficacy (VE) of the various vaccines are illustrated in the table below.
Waning immunity with time conferred by all the available vaccines, especially the inactivated vaccines, would mandate a booster dose. This is unlike the live attenuated measles and varicella vaccines which have a VE of 97 per cent and 98 per cent respectively with two doses, and will confer virtually lifelong immunity.
To begin with, the inactivated vaccines produced less protective neutralising antibodies, when compared with the mRNA vaccines.
Studies have also shown that the neutralising antibodies further declined six months after the second dose. Only 35.2 per cent of the vaccinees had neutralising antibodies above the minimal protective level. A third booster dose has been shown to increase the levels three to fivefold.
Israel has immunised their high-risk groups, the senior citizens and the immunosuppressed, and it is beginning to show results.
Thailand’s preliminary data has shown that two doses of Sinovac boosted by a third dose of the AstraZenca vaccine produce significantly higher neutralising antibody titers.
Malaysia needs to seriously consider this option, as emphasised in an earlier article. Only with a formidable herd immunity can we begin to see endemicity kicking in.
Meanwhile, the pandemic trinity of masking, distancing and hygiene, in tandem with ventilation, needs to be in place for a while longer to buy some time to suppress the transmission of the virus and protect the capacity of our health care services, while building a reserve surge capacity for any upcoming waves.
Smart and savvy risk communication that explains the transition to endemic Covid-19 is of paramount importance. In the past, the authorities have failed miserably with the basics of informing, educating and inspiring the rakyat.
Until and unless this strategic communication failure is addressed promptly and professionally, efforts to regain the trust of the rakyat will falter miserably.
- This is the personal opinion of the writer or publication and does not necessarily represent the views of CodeBlue.
