COVID-19 surge: Why has China not attained herd immunity despite high vaccination coverage?

Germany, South Korea, Vietnam are also reporting increase in COVID-19 cases despite extensive vaccination

By Vibha Varshney
Published: Wednesday 16 March 2022

China is once again in the grip of a COVID-19 wave. The country reported 1,952 new COVID-19 cases on March 15, 2022, according to data shared by National Health Commission of the People's Republic of China. 

The surge prompted authorities to lock down many places, including Shenzhen, which is a technology and business hub.

Shanghai suspended in-person classes and shut intercity bus services, while the northeast industrial centre of Changchun, which accounted for about 11 per cent of China’s total annual car output in 2020, was locked down last week.

With this, China joins other countries like Germany, South Korea and Vietnam that are reporting significant rise in cases despite extensive vaccination.

Such surges put a question mark on our understanding of herd immunity, a protection that the world had pinned its hopes on in the initial days of the pandemic.

It was hoped that COVID-19 infection and vaccination would provide immunity against the disease. Once a requisite number of people become immune, either through infection or vaccination, transmission of the disease would reduce and this would indirectly protect the rest of the population.

Herd immunity was recognised as a natural phenomenon in the 1930s when it was observed that after a significant number of children had become immune to measles, the number of new infections temporarily decreased.

According to WHO, the percentage of people who need to be immune in order to achieve herd immunity varies—it is 95 per cent for measles, 80 per cent for polio and so on.

It has been observed that once herd immunity is reached, the disease starts disappearing from the population.

However, the COVID-19 pandemic has turned our understanding of “herd immunity” on its head.

The actual number of people that needed to be immune was not known as COVID-19 is a new disease. A pre-print study published in the journal MedRxiv September 14, 2021 estimated that 93 per cent people will need to be immune to result in herd immunity.

Researchers in China used a model to study SARS-CoV-2 (delta variant) transmission and found that due to low efficacy of the vaccines used in the country, they are unlikely to reach herd immunity in 2021. The researchers wrote:

We estimate that, assuming a vaccine efficacy of 90 per cent against the infection, vaccine-induced herd immunity would require a coverage of 93 per cent or higher of the Chinese population.

Many countries currently facing outbreaks have achieved this level of vaccination. In the last seven days, the top three countries reporting the maximum number of cases are South Korea, Germany and Vietnam. 

In South Korea, 88 per cent of the people have received both the doses of the vaccine, in Vietnam, 79 per cent are fully vaccinated and in Germany 75 per cent of the people are fully vaccinated. In China, 85.5 per cent of the population is fully vaccinated. 

There can several reasons for this failure of herd immunity.

First, unlike other vaccines that prevent diseases, COVID-19 vaccines just protects from severe illness. Even vaccinated people fall sick, are asymptomatic and can transmit the virus.

Second, the immunity provided by the COVID-19 vaccines wane quickly. Vaccination began only in December 2020 and at best, we have efficacy information only for a year. A study published in The New England Journal of Medicine February 16, 2022 suggested that immunity in people with Pfizer’s mRNA vaccine started waning after six months. In people who were infected and vaccinated, immunity started waning after a year.

Third, this is the first time a vaccine has been used on adults who have poorer immune response compared to children. The younger population also continues to spread the disease.

Fourth, a large proportion of the global population is still unvaccinated and this is resulting in emergence of new variants. The vaccine can also be putting evolutionary pressure on the virus and lead to the development of new variants.

This is being considered to be the reason for recent surges in vaccinated countries.

The omicron variant was identified in November 2021 by researchers in South Africa. However, within some weeks, it was observed that this variant has three branches BA.1, BA.2 and BA.3.

Initially, BA.1 was dominant but at present, BA.2 is becoming more prevalent. This variant has been termed ‘stealth variant’. Though BA.2 does not cause more severe disease, it spreads more readily.

In these circumstances, it is unclear when we would be free from the risk of the disease. If we want to depend on herd immunity, we need to figure out ways to keep immunity above the herd immunity threshold. 

According to a study published in Frontiers of Medicine journal February 28, 2022, vaccination strategies need to adapt to this. For this, the authors suggested increasing vaccine coverage and providing timely booster doses. They also prescribe research to understand the best prime-boost vaccine combinations and immunisation strategies. 

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