As of late October, 92% of Singaporeans have completed the basic vaccination regimen of two doses. 79% of the population has also received at least one booster shot according to Statista. Even with the population’s high vaccination rate, questions have arisen regarding the threat of the XBB variant and the possibility of a newer strain emerging. Dr. Loh Jiashen, Infectious Diseases Specialist, Farrer Park Hospital sheds light on how we should understand the factors in play.
The XBB strain is a recombinant variant of the coronavirus, with the ‘X’ representing the hybridization of two constituent ‘B’ variants – BA.2 plus BA.4 or BA.5. The first signs of a rising XBB wave began showing around two months ago.
At around mid-October, the proportion of XBB strain infections started overtaking that for the previously dominant BA.5 strain.1 The same surge pattern has been witnessed across several other Asian countries.
A few media outlets have referred to an October 19, 2022 World Health Organization report stating that the XBB variant is highly immune-evasive.
This means that the virus has strong capabilities to bypass antibody defenses and jump from one host to another.
From an evolutionary perspective, XBB’s immune-evasiveness does not come as a surprise to Dr. Loh. Less virulent strains naturally die off in host bodies while more resilient variants leave their hosts to infect more individuals and multiply.
But despite being more contagious, it is noteworthy that the XBB strain does not seem to pose a significant threat to human life, especially among children, the vaccinated, and individuals who have contracted Covid-19.
High vaccination rates coupled with the fact that an estimated 60% of the population has contracted Covid-19 according to Singapore’s health minister2 means that most people have built up significant antibody defenses against Covid-19.
This creates significant environmental pressure on Covid-19’s evolutionary path.
Dr. Loh points out that studies on recent coronavirus strains have not shown one to be significantly more lethal than another. On the contrary, the number of deaths marked according to Covid-19 variants suggest that Omicron is actually less lethal than Delta.
However, this does not rule out the possibility of new variants becoming more potent
Current mask mandates generally only require people to mask up on public transport and in healthcare settings where human density and the risk of exposure to the virus are high.
With the relaxation of mask rules, viral permeability would increase, providing the virus opportunities to spread. This also means that new variants are likely to show up more frequently as the virus gradually mutates at an increased rate with increased transmission.
The number of Covid-19 cases rose slightly after mask rules were relaxed in August of this year. However, the latest XBB wave peaked soon after mid-October before dropping back to manageable levels.
The new case numbers in the most recent wave were still a far cry from the peaks that climbed beyond 19,000 earlier in February.
The initial rollout of the bivalent Moderna/Spikevax vaccine took place on October 14 of this year for those above 50 years of age, who received their last dose of a Covid-19 vaccine more than 5 months prior, or without minimum vaccine protection by the Ministry of Health’s definition.
This was followed shortly by the rollout to healthcare workers across the public and private sectors to help them guard against high exposure at work.
More than 28,000 persons had either received this booster vaccine or booked an appointment to do so for up to October 20.3
Of course, this begs the question of why an individual should vaccinate himself against original strains that are barely circulating in our population.
Combining the original vaccine with boosters has been scientifically proven to boost antibody counts in subjects, Dr. Loh explains.
This provides added protection against old variants, new variants, and even possibly variants that lie outside of the targeted spectrum.
For example, people who completed the original two-dose vaccination course and received booster jabs for the Beta strain were found to have a high level of antibodies against the Delta variant.
Therefore, there is scientific basis for BA.5 and BA.1 boosters to be included in the bivalent package for protection against future strains beyond the XBB variant.
If last year’s national vaccination programs were any indication, playing constant catch-up with the latest Covid-19 strain may stretch resources and logistical efforts thin in the long run. This is especially so when considering how the virus evolves faster than boosters can be produced.
That said, the current situation where there are high case numbers but low ICU admissions gives the healthcare system more time to respond and roll out boosters.
Dr. Loh opines that it may be more feasible for vulnerable populations such as the elderly and immunocompromised to stay on an up-to-date booster regimen instead, especially since boosters are not as critically important for young, healthy adults.
The international medical community is looking for a longer-term solution for Covid-19 by exploring the possibility of producing a universal vaccine.
Can such a vaccine be made? The short answer is that developing a universal vaccine is scientifically possible. Earlier studies on SARS-CoV-1 survivors who were vaccinated with the SARS-CoV-2 vaccine found that subjects had developed antibodies against even animal coronaviruses that had not entered the human sphere.
However, Dr. Loh admits that the hurdles to realizing this silver bullet are substantial. Even with influenza, vaccinations are still being rolled out on a seasonal basis. For now, the achievement of a universal coronavirus vaccine still remains distant
According to Dr. Loh, studies have shown that people who have contracted the BA.2 virus have greater protection levels of up to 75% against the BA.4 and BA.5 variants, compared with people who have only completed the original vaccination regimen having protection drop to as low as 40% or even 30% over time.
In other words, getting infected with recent variants in a vaccinated person does confer hybrid immunity that will protect from existing strains.
Nonetheless, it appears that getting boosted still provides an individual with a good chance of both avoiding Covid-19 and recovering from it.
As things stand, Covid-19 will likely play a less significant role in our lives in 2023. However, conditions may change to defy our expectations.
For now, it is still advisable to err on the side of caution whether you fall in the category of being vaccinated, boosted, or elderly.
References
1 https://www.japantimes.co.jp/news/2022/10/28/national/xbb-subvariant-tokyo-cases/
2 https://www.channelnewsasia.com/singapore/covid-19-herd-immunity-singapore-residents-cases-omicron-wave-2849511
3 https://www.moh.gov.sg/news-highlights/details/bivalent-moderna-spikevax-vaccine-to-be-made-available-to-healthcare-workers
