Is a booster dose necessary for COVID-19 vaccines in Hong Kong?

Written by: Dr David Owens

The short answer is no, at least not yet. It may be necessary at some point in the future. It is possible that people who have been given Sinovac will be offered a booster either with BioNTech or an alternative future vaccine. It is unlikely that people who have been given BioNTech in Hong Kong will need a booster, at least in the short term.

The announcement that booster doses will be given in many developed countries has been condemned by both the WHO and the majority of international public health experts. When only 1% of people in the poorest nations, including healthcare workers, have received a single vaccination, it is hard to justify the marginal benefits of boosting especially in the context of the risk of variants, if the epidemic continues to burn in countries without available vaccines.

In a recent article in Nature magazine, Professor Cowling argued for fractionation. His argument is that current vaccine doses should be divided up such that smaller doses of the most effective vaccines are given to the largest number of people. This theory accepts that on an individual level the immune response will be slightly lower but in population terms, the benefit will be greater in terms of reduced total mortality and reduced risks of variants. It is important to appreciate that just because a certain vaccine dose and schedule are chosen in a clinical trial, it does not mean that this is the only or even the best option. This has been clearly demonstrated in the UK. The decision to extend the timing of the second COVID-19 vaccine, beyond that recommended by the manufacturer, was subject to criticism at the time. There is now good evidence that the decision to immunize as many people as possible with a single dose, whilst delaying the second dose, saved many thousands of lives. In some ways, these simple ideas demonstrate the dilemma of public health. Ultimately in public health terms, acting in the interests of the majority increases the benefits to the majority whilst accepting inevitably that the minority may not have the optimal outcome for them (whether that be extra vaccination or quarantine). Vaccine inequality is a major threat to all because it increases the risks of new variants. This issue was discussed further in a recent Podcast with Professor Cowling.

So, are Covid booster shots a bad idea?

Answer: No, but they should be given in an evidence-based way that maximizes population impact.

The evidence for both population effectiveness and antibody response for the mRNA vaccines is excellent. We now know that BioNTech vaccine is significantly more effective than Sinovac in terms of the generated antibody response[2]. In OT&P patients we found that one BioNTech vaccine is roughly the same in terms of antibody response as two Sinovac vaccines. However, the second dose of BioNTech gives a significant boost. In a previous article, I argued that it may not be possible for Hong Kong to reach Herd Immunity by vaccination alone. This argument may not apply in countries with a very high uptake of effective vaccinations leading to high levels of immunity. In China, there are now high levels of vaccination almost exclusively with killed vaccines. We know these vaccines are good at preventing death and serious disease but less effective in controlling an epidemic in population terms. It seems very likely that the strategy in China will be to give a single booster of an mRNA or possibly next-generation vaccine in order to boost immunity levels produced by Sinovac and Sinopharm vaccines. It would certainly be possible for Hong Kong to do the same thing, although this decision will be based on increasing data and evidence of the effectiveness of boosting with a different vaccine. Early studies suggest that this strategy is likely to be effective.

In summary, we now have very good evidence about the effectiveness of different vaccines. We are beginning to accumulate evidence around which vaccines produce a better immune response, which vaccines are most effective against evolving variants and importantly we are beginning to explore the optimal dose and timing between doses to provide population benefit. It seems likely that boosters will be used in different ways. The traditional way is to boost previous vaccinations. We do not yet have good evidence of frequency, but it seems likely that mRNA vaccines, such as BioNTech, will last for at least one year and possibly much longer. In some situations, we may end up mixing vaccinations such that boosters with an mRNA (or next-generation vaccine) are used to increase the effectiveness in population terms of less effective vaccines (such as Sinovac). Finally, if we see a mutation which escapes current vaccines, we may give boosters with next-generation mRNA vaccines which are designed specifically to protect against new variants.



1. Cowling, B. J., Lim, W. W., & Cobey, S. (2021, July 5). Fractionation of COVID-19 vaccine doses could extend limited supplies and reduce mortality. Nature News.

2. Lim, W. W., Mak, L., Leung, G. M., Cowling, B. J., & Peiris, M. (2021, July 15). Comparative immunogenicity of mRNA and inactivated vaccines against COVID-19.

Topics: COVID-19

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