In an article published in the South African Medical Journal, Shabir Madhi, Professor of Vaccinology at Wits, argues that COVID variants have made the initial goal of attaining herd immunity no longer feasible, even for well-resourced countries. However, vaccine protection against severe COVID seems a more realistic path to normalcy.
In low and middle income countries (LMICs), the official COVID case estimates are likely grossly underestimated, Prof Madhi writes, due to a lack of testing coverage. Even in South Africa, the true number of COVID cases is likely in the region of 10 times the 2.39 million recorded through testing. The true number of COVID-related deaths in India is also estimated as 3.4–3.9 million, again 10 times the official count, and in South Africa it is likely three times the official figure of 70 388 in July 2021.
While New Zealand researchers have suggested that COVID eradication is feasible, it is likely a very long term goal if at all attainable. The herd immunity goal can be considered with the equation (p1 = 1 – 1/R0), where p1 is the proportion of immune individuals who will also no longer transmit the virus, and R0 is the reproduction rate, ie the number of susceptible individuals a single infected person can further infect. However, this ignores key aspects of the virus.
The problem is that the proportion of people that would need to be immunised to achieve herd immunity was initially calculated at 67%, based on an assumed R0 of 3, derived from the Wuhan strain’s R0 of 2.5 to 4. However, the Delta variant has an R0 of 6, meaning that to reach herd immunity, 84% of the population would need to be vaccinated. In South Africa, this would be 100% of the population aged over 12.
The emergence of SARS-CoV-2 variants, especially the Beta variant with the E484K mutation, showed that existing vaccine protection, including the Pfizer variant, can be degraded to an extent.
Studies have strongly suggested that neutralising and antibody titers are associated with mild to moderate COVID protection, while protection from severe COVID may be mediated by T-cell immunity.
Real world data showed that in Israel, with a world best immunisation of 61.6% using the Pfizer vaccine which produces the greatest antibody response, herd immunity appeared to be successful until an outbreak of the more transmissible Delta variant combined with waning vaccine effectiveness.
However, in the UK, excess death data showed that, even with a resurgence of cases caused by the Delta variant, there was a significant decoupling of deaths from cases. This points to the effectiveness of vaccines in preventing severe illness, as opposed to reaching herd immunity.
Vaccine rollouts have therefore not interrupted COVID transmission. Prof Madhi concludes that, based on an estimated R0 of 6 for the Delta variant, “it is unlikely that any country could have a sustainable strategy for durable high level of protection against infection by the delta variant. Mutations of the SARS-CoV-2 genome are likely to continue resulting in enhanced transmissibility, infectiousness and resistance to neutralising activity.”
He observes that the “UK approach seemingly concedes that the goal of herd immunity, even in a highly resourced setting, is unattainable.”
He adds that aspiring to reach herd immunity by wealthy countries comes at the cost of exacerbating vaccine inequality, which he says “is immoral.”
Antibody dynamics modelling suggests that a booster would be required every 2–3 years to protect against severe COVID, and every 6–9 months to protect against moderate disease. This is a challenging goal, and likely unattainable for most LMICs, especially given the slow rate of vaccination in those settings.
Source: South African Medical Journal
