Features
Herd immunity and vaccination
HERD IMMUNITY: A good analogy is protection of calves in a herd of wild buffalos from predation by leopards. A sizeable number of adult bulls and cows in the herd attack and repulse leopards. Once in a way, a leopard would succeed dragging a calf, but a large majority of calves survive to ensure the continuation of the species. (Picture courtesy HAP Channel: https://www.youtube.com/watch?v=igx_pr6ptAg&ab_channel=HAPChannel)
By Prof.Kirthi Tennakone,
National Institute of Fundamental Studies
(ktenna@yahoo.co.uk)
With the advent of coronavirus vaccines, the idea of herd immunity is gaining ground – but often misunderstood or considered something hard to fathom. Herd immunity means the resistance a community develops against an infectious disease, when a fraction of its residents above a threshold acquires immunity either by exposure to the pathogen or vaccination. Thus, achieving herd immunity could safeguard individuals who cannot be immunized for reasons of being too young, convalescent or because of inadvertent inaccessibility.
A good analogy is protection of calves in a herd of wild buffalos from predation by leopards. A sizeable number of adult bulls and cows in the herd attack and repulse leopards. Once in a way, a leopard would succeed dragging a calf, but a large majority of calves survive to ensure the continuation of the species. If leopards prey exclusively on buffalos, they might be starved into extinction. Buffalos and leopards live in the jungle because the latter also hunt other animals. Similarly, in absence of non-human reservoirs of the pathogen, herd immunity provides a way of controlling an infection causing an epidemic or a pandemic and the elimination of the causative agent.
History and theory of herd immunity
Epidemics originate when a pathogen invades a population devoid of immunity. Science fiction writer H.G. Wells in his novel, “The War of the Worlds”, says Martian invaders were not immune to earthly microbes and all died due to an infection. We are not so alien to viruses here and the ability to make antibody machinery to fight them are genetically imprinted in our bodies.
Even in olden days when precautionary measures remained completely unknown or misunderstood, maladies ended before everyone caught the infection. Those days, epidemics were considered divine punishments or expressions of anger of deities. The cause that receded them; attributed to prayers, rituals or offerings to the demons, has been in fact the natural herd immunity.
The Mahavamsa and the Elu Athanagalu Vamsa refer to a catastrophe during the reign of King Sri Sanga Bodhi (252-254 CE). According to the legend in the latter script; a demon named Ratharaksha came to Sri Lanka and cast a spell reddening the eyes of people who stared at it in fear. Many who looked at the eyes of those afflicted also developed red eyes and contracted the illness. Very high mortality thinned the population of the land and the distressed king, ritualistically confronted the demon driving it to exile. The version of the story in Mahavamsa is similar but implicate a female demon Ratarakshi. What is the infectious agent behind this outbreak? From the symptoms described and the extreme contagiousness implied, the illness that ravaged the kingdom seems to be measles. The herd immunity threshold of measles exceeds 95%. There was also a famine accompanying the epidemic. Presumably, malnutrition and absence of immunity greatly increased the measles death toll.
Ages ago people lived in isolated communities. Therefore, an infectious disease which decelerated and vanished after reaching herd immunity did not remerge until the immunized percentage was lowered by people born subsequently. Many epidemics, notably small pox and plague followed cyclic patterns for this reason. Later on, the establishment of vast human settlements and extensive migration, turned epidemics into pandemics and many diseases remained endemic. Historians have also argued that the consequent wider dispersion of diseases, boosted the immunity of the global human herd thereby escalating the population growth.
The idea of herd immunity was first introduced by the American veterinarian George Potter in 1917; he noted a cattle disease disappeared on its own when animals were not introduced to the herd from outside. He said disease resembled a fire which extinguished when all fuel has been consumed.
In 1919 bacteriologist W. Topley infected a few mice in a large colony with a germ. He observed the infection expanded, subdued and stopped after infecting only a certain percentage of mice. Further clarification of difference between individual immunity and herd immunity followed from the work of American statistician A.W. Hedrick. He studied the epidemiology of measles in United States 1900-1911 and concluded measles epidemics ceased when 68% of children under 15 years became immunised after contraction of the illness.
The idea of herd immunity was firmly established after invoking mathematics into epidemiology – mathematician turned physician Sir Ronald Ross pioneered the theme.
Ronald Ross, born in India 1857, received his education in the United Kingdom and returned to his country of birth after qualifying as a doctor. He joined the Indian Medical Service 1880 and worked in Bangalore badly infested with mosquitoes. At the time malaria was suspected to be associated with mosquitoes. Curious, Ronald strived hard to understand how it was transmitted. Mosquitoes in the place he lived has been a nuisance; he closed all stagnant pools in the vicinity of his residence and found the mosquito number falling drastically, but realized complete elimination would be an impossibility. When Ronald Ross was transferred to a station free of malaria, he declined to work in a locality free of malaria!
In 1895, Ronald Ross identified the malarial parasite in stomach of anopheles mosquitoes proving its mode of transmission. He was awarded 1902 Nobel Prize in Physiology for this work done in India.
Having found the cause of malaria; Ronald Ross determined to find a way to eradicate it and resorted to mathematics in attempting to find an answer. His remarkably insightful mathematical analysis revealed malaria could be eradicated by reducing the mosquito population below a threshold dependent on human population density, and the impossible task of destroying every anopheles mosquito was unnecessary. Following work of Ronald Ross, another physician A.G. Kendrick and biochemist W.O. Karnack both well versed in mathematics generalized Ronald Ross’s hypothesis, concluding the progress of infectious disease in a community depends on the average number of infected persons reproduced by one single carrier of the pathogen. If this number referred to as basic reproduction number (R) exceeds unity, the infection could expand into an epidemic whereas when the number is less than one the disease subsides after infecting a few. From statistics pertaining to the growth of an infection, the basic reproduction number can be estimated.
It is easy to see how an infection evolves depending on whether R is greater or less than one. Suppose 10 persons contracted with an infection with R=2 enters a susceptible population. On average, they pass sickness to 20 individuals and this 20 in return reproduce 40 cases – an endless series of ascending numbers. If R is less than one you obtain a descending sequence – implying cases die down.
Herd immunity threshold
Suppose a population of N persons includes a number M of individuals immune to a disease. The fraction of immunes in the population is M/N (M divided by N). From simple school arithmetic, it follows that the fraction of persons not immune (susceptible) is (1- M/N). In the presence of immunes, the basic reproduction number scale down proportionately to the fraction of the susceptible population so that the effective reproduction number is (1 –M/N) times R, written as (1-M/N) R. The threshold happens when the effective reproduction number is exactly equal to unity, implying (1 –M/N) R = 1 or equivalently M/N = 1 – 1/R. The fraction M/N given by the above formula, referred to as herd immunity threshold is normally expressed as a percentage. For example, measles being highly contagious, the basic reproduction number can take values close to 20. Setting R = 20 in the formula, we obtain M/N = 0.95. Expressed as a percentage, the herd immunity threshold for measles is 95. To protect a community against measles, over 95 percent of the population needs to be vaccinated.
Vaccinating a community to exceed the herd immunity threshold would not abruptly halt an epidemic. Although the incidence of the disease gradually decreases, vaccinations and containment measures have to be continued until positive cases disappear completely – smallpox was eradicated this way.
Can we achieve herd immunity to COVID-19?
Coronavirus vaccines have arrived sooner than expected – many countries including Sri Lanka expeditiously commissioning inoculation campaigns.
Vaccinations and continuous adherence to precautionary measures will undoubtedly tame the virus. However, it is premature to assume global herd immunity would follow and the pandemic will soon end.
According to some estimates an upper bound to basic reproduction number for COVID -19 is around 2.5. Formula M/N = 1- 1/R explained previously, imply that the herd immunity threshold corresponding to R = 2.5 is 60 percent. Vaccines may not be 100 percent efficacious. For an 80 percent effective vaccine, the above thresholds increase to 75 percent. The other question is how long the vaccine induced immunity would last. At the moment sufficient information is not available to decide how the duration of immunity will interfere with the herd immunity threshold and how often vaccinations need to be repeated.
If faster spreading variants of the virus take over, the basic reproduction number and therefore the herd immunity threshold will also increase. The variants may turn out to be more resistant to vaccines. Remodeling of vaccines to make them effective towards variants is technically feasible but would delay the immunisation protocols. The answer to the problem of variants and temporary immunity is speedy vaccination – obviously constrained by real world practicalities.
Decreasing trends of COVID -19 incidence
Many regions of the world have begun to see a decline in the number of COVID-19 cases and deaths – plausibly a combined outcome of preventive safeguards and immunity derived from exposure to the virus or vaccination.
Israel has given more coronavirus vaccinations per capita than any other country – around 50 percent given one dose and 35 percent both doses. Covid-19 cases are declining and the world is awaiting see the outcome of the Israel experiment.
The United Kingdom has vaccinated more than 30 percent of over 80s and noticed a dramatic reduction in COVID-19 related deaths in this group.
Prompt inoculation of a sizeable fraction of a community is not an easy task. We need to await patiently to see the effectuality of the vaccines.
Dependence of herd immunity threshold on preventive measures
The preventive strategies or so-called non-pharmacological interventions significantly reduce viral transmission thereby lowering basic reproduction number and therefore the herd immunity threshold. Wearing masks, social distancing, hand-sanitizing and ventilation are proven safeguards. There is some evidence and theoretical arguments to the effect that preventive measures not only reduce the risk of contracting the disease but those who catch the disease under such circumstances develop milder symptoms or recover soon, adding to the pool of immunes. Argument rest on inoculum theory of viral transmission, according which the intensity of the infection a patient develops depends on the number of virus particles to which he or she was exposed. Emphasizing this point authors of a recent article published in the prestigious medical journal Lancet appeal to the world to continue strict adherence to preventive measures. This is most prudent method to safeguard against new strains until vaccines are remodeled.
Vaccination priorities
Vaccine production, procurement and organization of immunization campaigns decide the rate at which a community could be vaccinated. These limitations necessitate imposition of priorities. The World Health Organization and individual nations have laid down priority categories. Everyone agree the first priority should be frontline health care workers. The second category the older persons (generally above 65) more vulnerable and at the risk of death after contracting the sickness. Those living under conditions of extreme congestion and poverty are also a priority group identified by WHO. The younger working class, although they are less susceptible to danger of COVID-19, needs to be vaccinated. The policy of neglecting the older group in favour of younger working class is not only unethical but also epidemiologically flawed. In modern societies the percentage older persons (above 65) and socially active are significant. They, being most vulnerable to contracting the sickness because of impaired immunity, if infected, could also be the super spreaders. Recent studies have confirmed the presence of super spreaders, who are mostly elderly patients carrying larger viral loads.
Social reaction to vaccination
Societies react to vaccinations within confines of two extremes: vaccine hesitancy and vaccine overconfidence. The former has prevented eradication measles in localities where the herd immunity threshold stands inordinately high. In some parts of the world, vaccine hesitancy confuses mass COVID-19 inoculation. The latter misconception equally undermines the control effort. Not wearing a mask or not adhering to social distancing because you got the jab is not right. Vaccines are not 100 percent effective and immunity sometimes slacken. People not wearing masks, believing assurance of safety after the jab creates social stigma for those not vaccinated to abandon the precautions.
Vaccines and non-pharmacological interventions will certainly suppress the virus. Rapid decline in reported cases in some parts of the world may be a sign of a distant herd immunity in that region – but what we want is a global effect. As WHO Director Tedros said, “Until we end the pandemic everywhere, we will not end it anywhere “
Features
‘Lord Edgware Dies’
It has been some time since I read an Agatha Christie, the plot of which I cannot remember. So, I was delighted to find on the shelves of a friend Lord Edgware Dies, which I had a vague memory of, but no certainty about who had done it.
When I read it, I found that my memory of who was probably the killer was correct, but I could not be certain and the red herrings Christie threw in were so diverting that until almost the very end I wondered if I had been wrong.
The plot is very simple. Jane Wilkinson, who is married to Lord Edgware, tells him that she is desperate for a divorce since she is in love with a very proper Anglo-Catholic peer, Lord Melton, but Edgware refuses to divorce her. She asks Poirot to talk to him, which he does, and is surprised to find that Edgware has told Jane he is prepared to give her a divorce. This was, after he had categorically refused, through a letter, which Jane said she had not received.
That night Edgware is murdered, after Jane had been to see him, or so the butler said, and also Edgware’s secretary. But Jane had been that evening at a grand dinner many miles away, where a dozen fellow guests could swear to her presence.
There was a solution however to the mystery of two Jane Wilkinsons, namely a skilful impersonator called Carlotta Adams who, in the opening chapter had impersonated Jane Wilkinson, who had also been at the performance. But when Poirot goes to see her, he finds that she had been found dead on the morning after Edgware had been killed, of an overdose. And in her bag was a gold case, with a strange inscription, that contained the drug, along with a pair of pince-nez.
Her maid said she had written a letter to her sister in America and posted it the previous night. Poirot asks Inspector Japp to get the letter, and a transcript is received from America, and in it the name of Edgware’s nephew Ronald Marsh is mentioned; he had taken Carlotta to dinner after her performance, with which the book opens, and had then set her a challenge. Japp arrests Marsh, but Poirot is not happy and asks for the original of the letter, which the sister sends him. That shows that a page is missing, and the tear is obvious, though that raises the question as to why it had not simply been cut.
Matters are further complicated by the fact that Marsh had gone in a taxi to the Edgware house, along with Edgware’s daughter Geraldine, in the interval of an opera which had previously seemed to provide them with cast iron alibis. Geraldine had gone in to fetch her pearls so that Marsh could raise money he needed, and thus had an opportunity to kill Edgware, as did Marsh, for the driver said he had got out of the taxi while waiting and gone into the house.
Marsh explained why he had gone to the house on the night of the murder as having followed Bryan Martin, an American actor, who had been in love with Jane, whom he saw go into the house with a key. But there was no one visible when he entered, and Geraldine almost immediately came down and they left together. And Martin too has become an object of suspicion to Poirot, for he had been to see him before the murders were discovered with a story of being followed by a man with a gold tooth – a story Poirot immediately realized was false when he was asked how old the man was, and was told he was young, for young people did not have gold teeth.
A heap of French money Edgware had got for a trip to Paris was missing, but since Marsh had no need for it after his cousin’s offer of help, Poirot deduces that it must have been taken by the butler, who has disappeared. Christie has stressed that he is astonishingly handsome, unusual in a butler, and Poirot notes a resemblance to Martin, so he thinks the mysterious man going into the house must have been him.
Incidentally, later Poirot assumes that Edgware’s change of mind was because he was involved in some scandal, and I believe Christie intends us to see the cause of this in his handsome butler, though this is not specified.
Meanwhile, Poirot has asked Japp to find out the provenance of the case found in Carlotta’s handbag, and it turns out to have been made in Paris, specially commissioned, and collected by a woman with pince-nez.
But then another murder occurs—that of another guest at the grand dinner, which provided Jane with her alibi. The victim is an actor who had been bemused when Jane, at a lunch, thought the Judgment of Paris referred to the city. He told Hastings he wanted to see Poirot, but was killed before he could get to the appointment. Poirot had rushed there when told about his request, but it was too late.
Meanwhile, Poirot has tried out the pince-nez on Edgware’s secretary, but she could not see through these. It was only a chance remark heard outside the theatre that led him to try them out on Wilkinson’s maid Ellis, a spare pair that had been appropriated for the night of the murders.
Poirot then lays things out, having summoned Martin and told him that he probably suppressed Edgware’s letter, as he had been dropped by then and he did not want Jane to marry another. But after teasing Martin, Poirot says that Jane was in fact the murderer, and she got Carlotta to impersonate her at the dinner while she went to the house and killed her husband. After meeting Carlotta later and checking with her through a call that she had
not been rumbled, Jane had gone ahead with the murder – she put veronal into her drink and the case with veronal into the handbag. She forgot to take out the pince-nez she had used earlier to imitate an American. Carlotta had registered as the American in a hotel and Jane had gone to see her, and there they exchanged identities. After seen the letter, she made use of it by tearing off the page that referred to her, and the S of She, so that the person who had challenged Carlotta to impersonate her seemed to be a man.
There is a coda in which Jane, condemned to death, writes to Hastings, still full of pride at her ingenuity hoping she will be remembered.
Features
Desilt reservoirs, learn from our ancient irrigation systems
by Prof. O. A. Ileperuma
Silting of reservoirs is a major problem today affecting our hydropower production and irrigation systems. The main Mahaweli reservoirs are silted to a considerable extent reducing the water holding capacity of them. Due to poor soil management practices, floodwaters deposit large amounts of silt in these reservoirs. When the Polgolla reservoir was fully drained about two years back, one could see mountains of silt in the lower reaches of the reservoir. A rough estimate is that 50% of the total capacity of these reservoirs has been lost to siltation. This is a serious issue which affects not only power and agriculture but also flood control.
Our ancient irrigation systems ensured that desilting of reservoirs took place under royal decree where all users of the reservoirs were ordered to carry out desilting of reservoirs during the dry season. The clay thus collected was used in making bricks for the construction of great stupas which dot the landscape of our ancient kingdoms. This ensured that the reservoirs had their full capacity filled with water for the next cultivating season. Our ancient kings were clever enough not to construct reservoirs by blocking main rivers such as the Mahaweli. A classic example is the Minipe left canal where they tapped only the surface water of Mahaweli. Even the bigger tanks such as Nuwara Wewa and Parakrama Samudraya were fed with minor rivulets. There were also other ingenious features in the cascade irrigation systems built by the ancient kings, such as mud sluice canals and forest reservations between the reservoirs in the cascade system. These reservations helped trap silt and remove excess nutrients, which could otherwise contribute to increasing salinity as water flowed from one reservoir to another.
- Parakrama Samudraya
- Kalawewa
- Kotmale
A classic engineering marvel is the former Yoda Ela, which carries water from Kalawewa to Nuwara Wewa and Tissa Wewa. It is 87 km long although the straight distance between these points is only about 40 km. The gradient of this canal is about 10 cm per km or 6 inches per mile. Yodha Ela functions as a moving reservoir and feeds about 4,600 hectares of paddy lands. It is a winding canal with about 120 smaller reservoirs on its way. It was constructed during the reign of King Dhatusena around 459 AD and later expanded by King Parakramabahu by connecting more reservoirs to the network. Unfortunately, during the Mahaweli project our modern-day engineers constructed a concrete canal replacing the winding path of this Yoda Ela also called Jaya Ganga. This effectively removed the ability of the old Yoda Ela to remove silt and nutrients. The bank of this Ela has wet zone trees such as jak and areca nut growing well. They take up the nutrients from the flowing stream making the water suitable for irrigation later.
Ancient Mesopotamian civilisations depended on dams constructed along the two main rivers, Euphrates and Tigris. After continuous irrigation of their fields over several thousand years, salinity of the irrigated lands increased making them unsuitable for agriculture. People died due to famine and this clearly illustrates the danger of blocking main rivers for agriculture. There is scientific evidence that the salinity of paddy soils in the Mahaweli C area is increasing.
We saw the devastation caused by Cyclone Ditwah. The sluice gates of the Kotmale Reservoir were opened, and Kandy and Peradeniya were flooded. If the reservoir had had greater storage capacity, couldn’t the opening of the gates have been delayed? This may not be an argument that modern-day engineers would readily accept, and I am not an irrigation expert. These ideas may well be naïve. But most of us tend to think of reservoirs mainly in terms of hydropower generation and irrigation, while their role in flood control receives much less attention. The question therefore deserves serious consideration. Could restoring lost reservoir capacity through desilting help improve our ability to manage extreme rainfall and reduce flood risks?
Desilting our reservoirs should be considered a national priority.
Features
Losing out to Ethiopia
Export diversification – Missing the wood for the trees – Part III
by Gomi Senadhira
In Sri Lanka, the word “Ethiopia” is often used as disparaging slang to describe individuals or areas experiencing extreme poverty, starvation, or severe economic hardship. This linguistic habit originated in the 1980s with the Western media coverage of the devastating Ethiopian famine of 1983-85. That media coverage shocked the world but also left an outdated and offensive global stereotype that the country is permanently starving. Much has changed since then. By now, with an annual growth rate of around 9%, it is the fastest-growing economy in sub-Saharan Africa. Ethiopia has also emerged as a highly competitive exporter and is challenging not only its competitors in the region but also countries like Sri Lanka. This article is on how Sri Lanka has lost ground to Ethiopia (and a few other countries) in the GCC markets for agricultural and floricultural products.
Sri Lanka – A Pioneer in the Agriculture and Floricultural Market in the GCC
As discussed in Part II of this article, by the mid-1980s Sri Lanka had established a strong foothold in the GCC’s fruit, vegetable, and floricultural market. Geographical proximity and well-established shipping and air links gave Sri Lanka a strong comparative advantage over Southeast Asian and African nations. Thailand, Vietnam, and Kenya were not even in the market. At that time, Ethiopia was experiencing (as BBC news reports described) “a biblical famine”.
The market was not very large, but it was lucrative and growing. Trade Minister Lalith Athulathmudali as well as the Chairman of the Export Development Board, Victor Santiapillai, who visited Kuwait (and the GCC countries), recognised the market potential for these products and encouraged us to continue with our work. The minister was particularly keen to further develop links between the market for these products, exporters, and his Export Production Villages (EPVs). So, it was becoming a successful case not only for export diversification but also for transferring gains from exports directly to rural households.
From Trailblazer to Tailender
As a result, even by the beginning of this century Sri Lanka had a larger market share than most of its competitors from Asia or Africa. But since then, our competitiveness has weakened significantly. The tables below provide a comparative snapshot of Sri Lanka’s performance vis-à-vis Thailand, Vietnam, Kenya and Ethiopia in the GCC market for vegetables, fruits and floricultural products. As illustrated therein, in 2001 Sri Lanka was ahead of Thailand, Kenya and Ethiopia in this small but rapidly growing market. Since then, we have fallen behind Thailand, Kenya and many other countries in that lucrative market. If this trend continues, Sri Lanka will fall behind Ethiopia within the next few years. (See Table 1)
In the GCC market for vegetables (covered in HS chapter 07), Sri Lanka was ahead of most other competitors in 2001. As illustrated in Table 1 , Sri Lanka had failed to develop this market, while Thailand, Kenya, and even Ethiopia had very efficiently increased their market shares. The GCC is a market to which Sri Lanka can supply some vegetables, like cabbages, by sea. It appears Sri Lanka had also failed to exploit this mode of supply.
We can see a similar trend in the market for fruits. Vietnam, Kenya, and Thailand have emerged as major players, while exports from Sri Lanka have staggered on slowly. In this segment, Vietnam has emerged as a leading player during the last twenty years and the GCC imports from Viet Nam have shot up from US$44 thousand in 2001 to US$346 million by 2024. In part one of these articles, I discussed the remarkable increase of jackfruit exports from Vietnam “…just $3 million in 2015 to an impressive $236.8 million in 2023” while most of our jackfruit production rots under the trees. This explains how countries develop their markets, geographically and product-wise. (See Table 2)
Sri Lanka’s performance has been weakest in the market for floricultural products (HS Chapter 06), which groups live trees, cut flowers, and ornamental foliage. When we first entered the market in the 1980s, the market was dominated by the Netherlands, and Kenya and Ethiopia were not even in the market. At that time, we identified the Gulf states as a market where Sri Lanka could have a dominant presence due to geographical proximity. Even in 2001, Sri Lanka was ahead of Kenya, Ethiopia, and Thailand. But by now, Kenya has emerged as the dominant supplier. Ethiopia is also expanding its market share and is the third-largest exporter. (See Table 3)
Missing the Wood for the Trees
In the mid-1980s, Sri Lanka first established its foothold in the GCC market. Since then, Thailand, Vietnam, Kenya, and even Ethiopia have moved well ahead of us and have become leading players. Why did we lag behind in our export diversification efforts in general and, more particularly, in the GCC market?
The reasons are very clear. After the initial attempts in the 1980s and early 1990s, Sri Lanka has not been proactively involved in identifying, developing, and promoting new products and markets, or protecting and further developing new markets already established. The focus has simply been on traditional exports: tea, coconut, cinnamon, and garments, while other products were almost ignored. In essence, we have been and continue to focus intensely on a narrow group of products and markets, and we have lost sight of the bigger picture.
(The writer can be reached at senadhiragomi@gmail.com)
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