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COVID-19 vaccination plans, herd immunity, and prospects for relaxation of public health measures in

As we pass the one-year anniversary of the declaration of the COVID-19 pandemic by the World Health Organization on 11 March 2020, we look back on a year of extreme disruption to our daily lives and wonder how much longer the disruption is going to last.

Vaccines have the potential to eliminate the need for social distancing measures, but only if a high level of population immunity is achieved. As I explain below, sustained elimination of COVID-19 is another option that may be considered by the Hong Kong government, and would be facilitated by higher vaccine coverage. If we would like to return to normal later this year, high community uptake of highly effective vaccines will be essential.

In the past year Hong Kong has fared much better than most parts of the world, due to timely action of its government and its people. We adopted universal masking long before most other parts of the world. We recognized that preventing importation of infections was important, introducing strict quarantines on arriving persons. We are a model for the effective isolation of cases, along with contact tracing and quarantine of close contacts, and this targeted measure has allowed more moderate social distancing measures compared to the societal lockdowns used in other parts of the world.

Now as we begin to use COVID-19 vaccinations locally, we can see hope for an eventual return to what we considered normal life before 2020, although we may have to settle for a “new normal”.

It is not easy to forecast the future, but here I lay out three broad strategies for the direction we could take in Hong Kong.

The first is the approach taken in some parts of the world, using public health measures to suppress widespread epidemics, relaxing the measures when case counts return to low levels, leading to cyclical use of social distancing measures and repeated waves of infections.

The second is a continuation of the general approach that has been implemented in mainland China, Macao, Taiwan, Australia and New Zealand, to force daily local case counts down to zero and then maintain them at zero.

The third approach would include a return to pre-2020 life with no need for public health measures to control COVID-19 transmission, but would only be safe for Hong Kong in the next one or two years if we can reach herd immunity through high vaccination coverage.

Each of the paths I describe have advantages and disadvantages, and none are perfect. My comments and evaluations are from the perspective of a public health scientist, who is not an expert in economics, social science, or politics.

We have so far relied on targeted measures (particularly quarantine of close contacts) along with moderate social distancing measures to control epidemics of COVID-19. Vaccination provides a less disruptive and more sustainable solution. There are two reasons why individuals might choose to receive COVID-19 vaccination.

The first is to protect themselves, and all available vaccines have been shown to be very effective in protecting against severe disease in adults. For example, while Sinovac had a reported 50% effectiveness against mild infections in healthcare workers, the effectiveness reported against mortality in this group was 100%. The second reason for people to receive vaccination is so that the community can achieve what is called “herd immunity”. I would like to explain this concept, first, and then come to the control strategies we can consider in the coming months and years.

Herd immunity

People line up to receive China's Sinovac COVID-19 coronavirus vaccine at a community vaccination center in Hong Kong, Feb. 26, 2021. AP Photo

“Herd immunity” is a concept describing a population that has a high enough level of immunity that a large epidemic will not be able to occur because immune persons act as barriers to the spread of infection. Herd immunity is not an all-or-nothing threshold to be crossed, but rather a useful concept to keep in mind when considering the approximate levels of effective vaccination coverage that might be necessary to feel confident that large epidemics cannot occur.

The potential for an infection to spread in a population depends on the contagiousness of infected persons, and the level of immunity in the population.

At the start of the COVID-19 pandemic, each new case was infecting an average of 2-3 others. This value is known as the basic reproductive number and is likely closer to 3 in densely populated Hong Kong. The objective of public health control measures is to reduce transmission and bring the reproductive number to below 1, and preferably as low as possible so that the spread of infection ends more quickly.

At HKU we have been tracking the reproductive number over time, and we have observed reductions in the reproductive number below 1 when effective public health measures have been implemented to control the second wave, the third wave and most recently the fourth wave. It is easier to control the spread of infection when a fraction of the population are immune, which of course is not the case with a novel infection such as COVID-19 to which there is no pre-existing immunity in the population.

A rough estimate of the level of immunity needed for “herd immunity” is given by the formula 1 – 1/R. For COVID-19 in Hong Kong, with a reproductive number of perhaps 3, the herd immunity threshold would therefore be somewhere around 2/3.

A simple way to think about it is that for a reproductive number of 3, a case would ordinarily infect an average of three others. But if 2 out of the 3 persons are immune and can’t be infected, that leaves just 1 infected, for an effective reproductive number of 1. In this scenario, as long as at least 2/3 of the population are immune, large epidemics could not occur.

This formula provides only a rough estimate because different segments of the population might contribute more to transmission.

For example, there could be variation in transmission in different age groups or occupational groups. Achieving higher levels of immunity in higher-transmission groups would then be particularly important in achieving herd immunity. A population which has reached herd immunity might still experience small outbreaks in pockets of the population that have lower levels of immunity, but those outbreaks would tend to come to an end naturally even without control measures, without a large number of infections occurring.

Herd immunity can be achieved through vaccination or through natural infections. In the past year less than 1% of the Hong Kong population have been infected, meaning that natural infections have a minimal contribution to population immunity here. If a herd immunity threshold for COVID-19 is around 2/3, it would require higher than 67% vaccination coverage, because COVID-19 vaccines are not 100% effective in preventing infection. We could reach 67% immunity in the population through 70% coverage with a vaccine that has 95% effectiveness, but it is unlikely we would be able to reach herd immunity if we use less effective vaccines.

It is important to note that herd immunity will need to be maintained. It is not a final destination like the finishing tape in a marathon. One reason for this is because immunity tends to decrease over time, unless boosted by vaccination or infection. Another reason is because of the possibility for variants of the COVID-19 virus to emerge which could escape existing population immunity to some extent. Population immunity to COVID-19 could be boosted by readministering vaccinations from time to time.

It is also important to note that herd immunity does not have a single numerical threshold, it is a concept describing the level of population immunity that should restrict any outbreaks to a small size. At higher levels of population immunity, the risk of even small outbreaks would be lower still.

Option 1. Aiming to protect public health and the healthcare system with intermittent use of public health measures (“suppress and lift”)

Residents line up at the temporary testing center for COVID-19, inside the lockdown area of Cannon Street, Causeway Bay, March 17, 2021. EYEPRESS Photo

Now onto the possible strategies Hong Kong could consider adopting by the latter half of 2021. The first strategy is “suppress and lift”, in which public health measures are implemented when community transmission of COVID-19 reaches a higher level, and threatens to overwhelm the healthcare system. The same measures can be relaxed when daily case counts reach a lower level. Transmission will then increase again, and so this strategy tends to result in peaks and troughs in case counts over time. Keeping public health measures in place when case numbers are at a low level can lead to elimination (the second strategy, below), but some governments may find it difficult to justify maintaining these measures when case counts are low.

It may not be necessary to quarantine arriving travelers if electing for a suppress-and-lift strategy in the longer term, because imported cases that occur during a suppression phase (with effective social distancing measures) are unlikely to spread when transmission is being suppressed, while imported cases that occur while community cases are increasing are just like adding drops into an existing ocean of cases. Nevertheless, some locations using the “suppress and lift” approach have chosen to require quarantine of arriving travelers, and one reason for this is to delay the importation of new variants.

Suppress-and-lift could be less desirable than the other two strategies I describe below, because of the health impact of periods of higher case numbers, and the social and economic impact of repeated public health control measures, while the lack of sustained elimination of cases would prohibit membership of any travel bubble among other locations that are aiming for elimination. Repeated intermittent application of social distancing measures would likely be needed until herd immunity were reached through a combination of natural infections and vaccinations, and if the suppression measures are timely and effective, as they have been in Hong Kong, it could take many years to achieve herd immunity through natural infections.

Once population immunity begins to increase with vaccination, suppression becomes easier, and suppression phases could be shorter while relaxation (lift) phases can be longer, but the cycling of suppress and lift could be continued until herd immunity is reached. If Hong Kong were to choose this approach, we might expect larger gaps between subsequent waves. We could gradually boost immunity through vaccination, initially with the use of BioNTech and Sinovac vaccines, and perhaps in 2022 or 2023 by further booster vaccinations.

While this policy is in place, individuals could choose to receive vaccination to protect themselves, and it would be particularly beneficial to arrange vaccination of elderly and other vulnerable groups in the population. COVID-19 vaccinations can reduce the risk of infection, and any breakthrough infections that still occur in vaccinated persons tend to be less severe on average.

However, older adults with serious medical conditions are the most vulnerable in our community but may not themselves be recommended for vaccination because of their poor health. Herd immunity through vaccination would be able to protect these vulnerable people even if they themselves are not able to be vaccinated.

Option 2. Aiming for sustained elimination of COVID-19

The second possible strategy is elimination. In Hong Kong we aspire to achieving elimination, and this strategy may be adopted locally for the next 12 months and possibly longer. Mainland China, Macao, Taiwan, Singapore, New Zealand and Australia also provide models for this approach. The aim of elimination is to force daily local COVID-19 infections down to zero, and then keep them at zero.

COVID-19 outbreaks may occur from time to time, but must be aggressively stamped out when they do occur.

An important component of elimination is strict quarantine of incoming travelers, and a 14-day quarantine period has so far been sufficient in Taiwan, Singapore, New Zealand and Australia, while mainland China have a minimum of 14 days quarantine with extended monitoring for some travelers after the first 14 days.

In Hong Kong and Macao, the quarantine period for arriving travelers has been extended to 21 days. As David Webb has noted, prevention of within-quarantine transmission to other quarantined persons or to staff of quarantine facilities is critical, and this particular loophole has led to a number of outbreaks in Australia and New Zealand. Other loopholes may also need to be kept closed, if we are to succeed in elimination.

Once local infections have reached zero, social distancing measures can start to be relaxed. If or when community cases reappear, rapid and aggressive action could minimize any resulting outbreaks, and allow a quicker return to zero.

Increasing uptake of COVID-19 vaccinations will aid elimination efforts, because this will suppress the size and impact of any COVID-19 outbreaks that do occur. As vaccination coverage increases in Hong Kong, we might be able to sustain elimination while relaxing some social distancing measures, because immunity in vaccinated persons will also help to reduce transmission in the general community.

Elimination would minimize the direct health impact of COVID-19, not only in terms of mortality but also recognizing that a minority of survivors have long-term symptoms, referred to as “long COVID”. Elimination would also allow us to participate in travel bubbles with other locations that are aiming to sustain elimination.

Mainland China would be likely to follow this approach for at least another year if not longer, due to the challenge in vaccinating the majority of its 1.4 billion population and the risk to public health of allowing widespread COVID-19 transmission in the absence of herd immunity.

Singapore, Taiwan, Australia and New Zealand might also continue to aim for elimination, at least until they are confident that they have reached herd immunity through high levels of vaccination coverage with highly effective vaccines.

However, the drawbacks of this approach include the economic consequences of retaining the quarantine of inbound travelers, and sporadic use of social distancing measures to deal with outbreaks. It may not be necessary to keep the ban on visitors in place, as long as visitors are also subject to quarantine, and the current 21-day quarantine might be reduced back to 14 days if it were determined that 14 days were sufficient to minimize the risk of importations of infections, as it has been in Taiwan, Singapore, New Zealand and Australia.

It is unlikely that proof of receipt of vaccination would allow a traveler to avoid being quarantined, since vaccinated persons can still be infected and transmit infection onwards. Falsification and forgery of vaccination passports could also become an issue. Since Hong Kong currently insists on quarantine for individuals from locations such as Taiwan, Singapore, Australia and New Zealand while there are zero local cases there and therefore close to zero risk of infection in a traveler, it is difficult to envisage allowing quarantine-free entry for vaccinated persons that have a greater than zero risk of infection from other parts of the world. However, if infection risk was considered when specifying quarantine duration, it might be possible to require a shorter quarantine period for individuals from low-risk areas particularly if they have been vaccinated.

Elimination could be maintained as a strategy in the medium term, as long as continued quarantine of inbound travelers is considered tolerable. Travel bubbles could be established to allow quarantine-free travel between locations succeeding in elimination, for example between mainland China, Macao and Hong Kong.

One final comment on the elimination strategy is that local vaccination coverage may not reach a high level while perceived risk of COVID-19 remains low. We have recorded 10,000 confirmed cases here in the past year, and continued use of the same measures with the experience and capacity gained in the past year encourages us to hope for fewer than 10,000 cases in the coming 12 months. It is conceivable that some individuals may not see the need to accept the low risk of side effects from vaccination when they might not expect to require the protection conferred by vaccination because of the low risk of infection in the community while elimination is succeeding. At the time of writing the fifth wave appears to have started, and increasing perceived risk of infection may stimulate higher vaccination uptake.

Option 3. Aiming for a return to pre-2020 behaviors

An alternative approach, likely to be adopted within the next 3-6 months in Europe and the United States is a return to pre-2020 life by no longer applying COVID-19 control measures after a high level of immunity has been reached in the population through vaccinations and/or natural infections.

If this approach were applied in Hong Kong, perhaps later in 2021, it would mean no more social distancing, no more school closures, no more closures of bars and restaurants, no more quarantine for travelers or ban on visitors, and no more isolation of mild cases, or contact tracing and quarantine of close contacts. Some less disruptive measures might be retained, such as the use of face masks in crowded public areas, and routine regular COVID-19 testing of healthcare workers particularly those interacting with vulnerable groups.

Doctors and nurses wait to receive China's Sinovac COVID-19 coronavirus vaccine at a community vaccination centre in Hong Kong, Feb. 23, 2021. Frontline workers and high risk people are the first in line to be vaccinated in the territory. AP Photo

This strategy sounds attractive but is not possible right now in Hong Kong because of the threat to public health and the healthcare system that would be posed by widespread COVID-19 transmission.

In the United States, almost 500,000 COVID-19 deaths have been recorded in their population of 320 million. In Hong Kong, an uncontrolled COVID-19 epidemic could lead to millions of infections and tens of thousands of deaths, far more than the average of 850 deaths per year from influenza.

We can envisage this if we multiply up the 10,000 cases so far (likely corresponding to 40,000 or so infections) with 200 deaths – with high transmissibility of COVID-19 we would expect millions of infections to occur in the absence of control measures.

The situation would be even worse if the healthcare system has insufficient capacity to take care of a very large number of ill persons, as has happened in other parts of the world in the past year. And while most COVID-19 deaths occur in the elderly, infections can be serious in other age groups and a minority of survivors can suffer from “long COVID”.

Although this strategy cannot be considered yet, it is possible to envisage this strategy being adopted later in 2021, particularly if a high vaccination coverage can be achieved. To date, less than 1% of the local population have been infected naturally, and so herd immunity would only be achieved through vaccination in the short term. In order to reach herd immunity through vaccination, it would be necessary to achieve high vaccination coverage with highly effective vaccines.

If we achieve a high vaccine coverage but fail to achieve herd immunity, perhaps because a substantial fraction of vaccinated persons receive a less effective vaccine, it might not be safe to relax public health measures. This is because without herd immunity, COVID-19 infectious would be able to spread. While vaccinated persons would have a lower risk of infection and a much lower risk of severe infection or death if infected, some of the most vulnerable people in our community will not be protected by vaccination. Moreover, despite the risk reduction provided by vaccination, if a large number of infections occur some will be serious or even fatal.

While Hong Kong has purchased 7.5 million doses of BioNTech vaccine, in the form of 1.5 million 5-dose vials, other countries have discovered that each vial actually contains enough vaccine for 6 doses. The European Medicines Agency has explained how to extract 6 doses from each vial. In Hong Kong, adopting this approach would provide up to an additional 1.5 million doses of BioNTech, and enough for 4.5 million persons – 60% of the population – to receive this highly effective vaccine.

Another possibility, currently not being considered, is increasing the use of more effective vaccines in Hong Kong, i.e. purchasing more vaccines from BioNTech or Moderna, and limiting the use of less effective vaccines. Greater use of highly effective vaccines could allow herd immunity through vaccination alone, without the risk posed by a community epidemic.

A trolley carrying Pfizer-BioNTech coronavirus vaccines is transported to a warehouse after being unloaded from a Cathay Pacific cargo plane at the Hong Kong International Airport in Hong Kong, Feb. 27, 2021. EYEPRESS Photo

The highest vaccination coverage we could currently achieve would be around 85%, because there are around 1 million children in Hong Kong who cannot receive COVID-19 vaccines and no COVID-19 vaccines are currently approved for use in children. Vaccine trials are underway in children, and it is likely that existing or new vaccines will be available for children later in 2021.

If herd immunity can be reached through vaccination, the advantages of this strategy include the opportunity to relax all public health measures currently in place to control COVID-19. However, it would be necessary to accept the potential public health impact of occasional small COVID-19 outbreaks in the same way that influenza seasons are a part of life.

All sectors of the economy could recover, including tourism, although Hong Kong residents might need to undergo quarantine to enter any locations that were still following elimination in the coming months or years, such as perhaps mainland China, Macao, Taiwan, Singapore, Australia or New Zealand. Visitors from those locations could be welcomed, although they might need to undergo quarantine on their return home.


I have outlined three broad strategies that could be followed. Sooner or later everywhere in the world will switch over to the third strategy, but some locations may continue to follow the first or second strategy for a while longer, to protect their communities and healthcare systems from the health impact of large COVID-19 epidemics.

It would be reasonable to switch to the third strategy once herd immunity is achieved either through natural infections, or vaccination, or a combination of both, although some locations might choose not to switch to this strategy even after herd immunity has been reached, if other strategies are considered to have greater benefit.

I am not sure which strategy will be in place in Hong Kong at the end of 2021. There are advantages and disadvantages to each, and the third strategy in particular may not be feasible unless we have high vaccination coverage.

If a lower level of immunity is conferred by some vaccines, it might be possible to use booster vaccinations at a later date in recipients of those vaccines as a way to reach herd immunity. In the U.A.E., third doses of Sinopharm vaccine are already being given to some individuals to boost immunity.

However, if the third strategy is chosen, while mainland China continues to aim for sustained elimination, Hong Kong residents may continue to be required to undergo quarantine when traveling to mainland China. This would pose difficulties for hundreds of thousands of residents who have been accustomed to crossing the border on a daily basis prior to 2020. China will ultimately switch to the third strategy, but perhaps not within the next 18 months.

Therefore, it is possible to envisage Hong Kong opting for sustained elimination for some time, even if we are able to achieve herd immunity through vaccination.

Author: CitizenNews Op-ed | Publish Date: 18.03.21 | Last Update Date: | 2021-03-18 10:28:50

By Professor Ben Cowling, School of Public Health, The University of Hong Kong

Source: here


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