Importance of Herd-immunity
Khairy's Comments
The Borneo Post: Khairy says no longer using 'herd immunity' as Covid-19 likely here to stay.
https://www.theborneopost.com/2021/06/17/khairy-says-no-longer-using-herd-immunity-as-covid-19-likely-here-to-stay/
"Khairy made these remarks during a webinar organised by the Oxford & Cambridge Alumni Network Malaysia, adding that he now preferred to use the term ‘immunise’, and recognised that vaccines were not a guaranteed solution to the pandemic."
It is dangerous for a powerful minister to misunderstand the meaning of herd immunity. Herd immunity is the state where immunisation efficiency is enhanced, by having people in a herd immunised. A herd is a group of people close together.
Effectiveness of Herd Immunity
Individually, the effectiveness of immunity is only 90%, but together, the effectiveness of the immunity is increased to 99.99%, if 80% of the herd is immunised. With such high effectivess, there is no need for any preventive methods any more. No mask or testing required any more, even if Covid19 is endemic. It will not be able to infect people rapidly.
You can see it happening to the Swine Flu. Despite it being endemic, its spread is short and quickly stopped, because many people were already immunised against Swine Flu.
Immunised here means, either recovering from Covid19 or vaccinated, similarly for Swine Flu. The fact that nobody bothered to correct Khairi shows the lack of knowledge of Malaysians on herd-imminity.
The statements in quotes and in italics, are from:
"While not every single individual may be immune, the group as a whole has protection. This is because there are fewer high-risk people overall. The infection rates drop, and the disease peters out."
"Researchers think that the R0 for COVID-19 is between 2 and 3. This means that one person can infect two to three other people. It also means 50% to 67% of the population would need to be resistant before herd immunity kicks in and the infection rates start to go down."
If R0 is 2, one person infects 2 of his neighbours at one time. If half of them cannot be infected because they are immune, only 1 is infected, making the R0 to be only 1. Infection can still go on. If higher than 50%, R0 should be less than 1, so the infection will die down as it cannot continue to infect subsequent neighbours.
Estimating Vaccination Rate for Effective Herd Immunity
Here you assume 50% are completely resistant, because each vaccination is 100% effective, but this is not the case for Covid19 vaccines. Only around 90% for each vaccination.
For example, in a herd of 200 people, if you vaccinate 50% of them, which is 100, only 90 is resistant to Covid19. So there are 110 which are not completely resistant, so the R0 will be larger than 1.
If vaccine is only 50% effective, you need to vaccinate 2 out of 2 neighbours, i.e. 100%, i e. everybody, if R0 is 2. So vaccine effectiveness needs to be higher than 50% so that we do not need to vaccinate everyone.
If vaccine effectiveness is 67% or 2/3, if you vaccinate 3 people, only 2 will be resistant. If R0 is 3, i.e. an infected person can infect 3 people. but because 2 are resistant, only one more is infected, making the effective R0 to be 1. It means that if Covid19, has an R0 of 3 on average, and we have a vaccine that is only 2/3 effective, we need to vaccinate more than 67% of the herd or groups of people in close proximity to each other.
You can apply this understanding to estimate the actual vaccination required for current vaccines. Experts like to quote various figures from 75-90% vaccination rate, but I believe it is an exaggeration but it is a safe estimate. Most of them do not take into account the natural immunity of people who already got the Covid19 but are not symptomatic, i.e. symptoms cannot be detected or easily masked by pain killers.
My own estimate is only 50% vaccination rate but experiences in UK shows that this is not true. It could be due to the emergence of the VOC, especially the Delta variant, or Indian virus. Its attack rate must be must higher than 90%, and average infectivity rate, R0, is higher than 3.
Attack Rate vs Infective Rate, R0
My definition of attack rate is slightly different from the above reference, but my explanation is based on the comments by experts in their announcements, especially in Hong Kong.
The attack rate for Covid19 was 90%. Which means that in an enclosed space where one infected person is present, out of 100 people in the same room, 90 will be infected. So the RO is 90. In fact, if there are 100,000 people, 90,000 will be infected making the R0 90,000.
But it is unusual for people to cluster in such large groups, so the official R0 is the behavioural patterns of the community and is just an average value.
Vaccinating the Herd First
The purpose of vaccinating is not only to prevent the spread of a disease but the most important is the prevention of deaths. Vaccines are most effective at preventing deaths, much more than preventing infection. The infection rate for vaccinated people is quoted as 60% but it could be those who are infected. That is, the chance of people who are vaccinated and yet still infected is 60%. But only 90% of those vaccinated will be infected. Deaths of vaccinated people are quoted as around 98% so much higher than infection prevention.
That is why the first priority is to vaccinate the more vulnerable people, those with morbidities and those who are older than 65 years old. However, we cannot ignore the herd immunity in preventing infection. That is why the first priority is actually the medical personnel because it is important for them not to be disabled in any way while combating the disease. Also, they are in contact with the most number of people who are infected with the disease, so vaccinating them first satisfies the need for herd immunity.
Of course, if there is limited vaccines, the first priority with be for the vulnerable first, even among medical personnel and front liners. In fact, this is done. Only medical personnel who are meeting Covid19 patients were the highest priorities. Unfortunately, many medical personnel died when they are still exposed to Covid19 in other medical departments because the patients were either lying or not knowing about their exact conditions.
So the priorities tend to be front-liners, then vulnerable. Vulnerable people are not the majority so we have a better chance of vaccinating them quickly than just vaccinating them or just the herd. Active people are much more than vulnerable people and many of them can survive Covid19, so not really necessary to vaccinate them first.
Of course, even among the vulnerable people, priority should be given to those who lead an active life and need to go out and meet people. So ignoring vulnerable and go straight to herds is unwise and not practical.
However, the third priority should be on those who are active and vital to the economy of the nations. This is where Malaysia had made a blunder. Even now, I notice that young and healthy people are given priorities over those who are actively meeting people, such as journalists, cashiers, taxi drivers and delivery personnel. Not to mention the factory workers in essential industries, who are causing more than 90% of the Covid19 cases in Malaysia, and therefore even those in community cases are caused by work-places cases.
However, herd priority is also a priority. So the advise of our expert, Dr. Malina in vaccinating the whole family at one appointment is good for herd immunity. Dr. Malina quoted compassion as the reason, but efficiency, reduction in infection risk and more herd-immunity, are additional reasons. You see, a herd is a group of people coming into contact with each other.
In fact, this priority for herd-immunity can be extended further, by prioritising, places with points of entries such as Airports, Border Posts, Kuala Lumpur, Padang Besar, Penang, Johor Bahru, Kota Kinabalu, Kuching, Miri, Sandakan, Tawau, first.
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