IELTS Academic Reading Practice Test 17

A The systematic prevention of infectious disease by vaccination is widely regarded as one of the most consequential applications of modern science. Its origins, however, reportedly predate any modern understanding of immunity by many centuries. Inoculation against smallpox using material from infected individuals — a procedure later known as variolation — was practised in China, India, and parts of Africa from at least the early second millennium, with mortality rates a fraction of those caused by natural infection. The technique was introduced to Western Europe principally through the advocacy of Lady Mary Wortley Montagu, who is said to have observed the practice while resident in Constantinople and to have had her own children inoculated in 1718. Her public correspondence promoting the technique attracted considerable controversy in early-eighteenth-century London, although uptake among the aristocracy gradually spread.

B Edward Jenner's 1796 demonstration that material from a related disease — cowpox — could provide protection against smallpox without the dangers of variolation is widely regarded as marking the foundation of modern vaccinology. Jenner inoculated the eight-year-old James Phipps with cowpox material taken from the dairymaid Sarah Nelmes and subsequently demonstrated that the boy was immune to smallpox infection. The published account, refused initially by the Royal Society, appeared in 1798 and is widely said to have been rapidly adopted across Europe, eventually contributing to the global eradication of smallpox declared by the WHO in 1980. The term vaccination itself is widely traced to the Latin vacca, meaning cow, in direct reference to the cowpox source of Jenner's protective material.

C A second great wave of vaccine development followed the germ theory of disease established by Louis Pasteur and Robert Koch in the 1870s and 1880s. Pasteur is credited with producing effective vaccines against chicken cholera, anthrax, and most famously rabies. The rabies vaccine, used to treat a nine-year-old boy bitten by a rabid dog in 1885, was developed in collaboration with the physician Émile Roux. The success is widely said to have transformed public attitudes toward vaccination and led to the establishment of the Pasteur Institute in 1888, which became a centre of vaccine research and production for the following century. By the late nineteenth century the principle of attenuation — weakening pathogens so that they trigger immunity without causing disease — was widely said to have been firmly established as the dominant route to new vaccines.

D The twentieth century saw an extraordinary expansion of vaccine availability. Vaccines against diphtheria, tetanus, pertussis, polio, measles, mumps, rubella, hepatitis B, Haemophilus influenzae type b, pneumococcus, and rotavirus all entered widespread use over the period. The development of the killed-virus polio vaccine by Jonas Salk in 1955, and the subsequent live-virus oral polio vaccine by Albert Sabin in 1962, came, in many researchers' view, to symbolise the public-health potential of vaccine programmes. Smallpox was eradicated in 1980; rinderpest, a cattle disease, in 2011. Polio elimination has been achieved in most of the world although the disease persists in small endemic foci. Childhood immunisation schedules, gradually expanded by national authorities, are widely credited with reducing infant and childhood mortality across virtually all income groups during the post-war decades.

E Recent decades have, according to many vaccinologists, seen rapid technical progress. Recombinant vaccines, produced from cloned subunits of pathogens rather than whole microbes, have advanced safety and shelf life. Conjugate vaccines, in which weak antigens are chemically linked to stronger ones to boost the immune response, have produced effective vaccines against bacterial pneumonia and meningitis in young children. The 2020 emergency authorisation of mRNA vaccines against COVID-19 was, in researchers' view, the culmination of more than two decades of methodological development, and has opened the field to a generation of mRNA-based vaccines against other diseases now in active development. Several research groups have extended the mRNA platform to candidate vaccines for influenza, respiratory syncytial virus, and a number of cancers, with early clinical trials underway.

F Vaccine hesitancy has emerged as a significant public-health challenge. Despite consistent evidence of vaccine safety and effectiveness, surveys in many countries generally show declining rates of confidence in immunisation programmes. Measles, for example, has resurged in regions where vaccination coverage has fallen below the threshold required for herd immunity. The challenge of maintaining public trust in vaccination programmes is widely said to have become as important to disease control as the underlying biological development of the vaccines themselves.

A Among the most striking findings of modern comparative biology is what researchers describe as the near-universality of sleep. Despite enormous diversity in the lifestyles, habitats, and brain architectures of animals, virtually every species so far studied appears to require periods of behavioural quiescence resembling sleep. The fundamental purposes of sleep remain, in researchers' view, incompletely understood, but its evolutionary persistence across nearly all multicellular animals is widely said to strongly imply a function that natural selection has not been willing to dispense with. Some animals are, however, widely said to push the limits of how briefly or how intermittently sleep can be taken without obvious harm, which is one reason the underlying purpose remains contested.

B The defining behavioural features of sleep are identified as reduced movement, characteristic body posture, raised threshold for stimulation, and a homeostatic response — that is, sleep loss leads to compensatory increased sleep when conditions permit. Neural correlates typically differ greatly between species: mammals and birds reportedly exhibit distinct stages including a state analogous to rapid-eye-movement sleep; reptiles generally show simpler patterns; while invertebrates such as fruit flies, octopuses, and even the jellyfish Cassiopea are widely said to show convincing sleep-like quiescence without comparable brain structures. The convergence of these distinct neural architectures on the same functional behaviour is widely treated as a sign that the underlying need for sleep is older and deeper than any particular brain design.

C The variability in how sleep is distributed across the day is, in many biologists' view, remarkable. Cetaceans — dolphins and many whales — reportedly sleep one brain hemisphere at a time, allowing them to surface for breath and maintain awareness of predators. Migratory birds engaged in trans-oceanic flights such as the alpine swift have been shown to sleep in short bursts while flying, with brief periods of unihemispheric or even bihemispheric rest documented during multi-week journeys. Some grazing mammals typically stand while sleeping, others sleep in deep burrows, and many small mammals reportedly sleep over twenty hours per day. The diversity of arrangements is widely said to reflect the diverse pressures of predation, foraging, and thermal regulation that shape every species' daily cycle.

D Proposed functions of sleep generally fall into several broad categories. Restorative theories typically propose that sleep allows cellular maintenance and waste-clearance, including the glymphatic flushing identified by Maiken Nedergaard in the past decade. Synaptic-homeostasis theories, developed principally by Giulio Tononi at the University of Wisconsin, generally propose that wakefulness progressively strengthens synaptic connections and that sleep is required to renormalise them — what some researchers describe as a kind of nightly garbage-collection of memory traces that would otherwise saturate. Energy-conservation theories observe that sleeping animals expend less energy and so may simply be hiding from predators in periods when foraging is unprofitable. These accounts are not necessarily exclusive, and many researchers now suspect that sleep serves several of these functions simultaneously.

E The evolution of sleep remains, according to many researchers, substantially mysterious. The fact that simple animals such as the nematode worm Caenorhabditis elegans reportedly show clear sleep states is widely said to imply that sleep is an ancient trait, probably present in the common ancestor of all bilaterian animals at least 600 million years ago. The molecular signatures of sleep are widely regarded as remarkably conserved: many of the same neurotransmitter systems, transcription factors, and even individual genes are reported to govern sleep across species separated by hundreds of millions of years of independent evolution. This deep conservation is taken as one of the strongest arguments that sleep performs some genuinely indispensable cellular function rather than being a contingent feature of particular nervous systems.

F The behavioural ecology of sleep is widely said to continue producing surprises. Antarctic chinstrap penguins, observed by Daniel Mancia and colleagues in 2023, were found to sleep in thousands of bouts of only seconds each over the course of a day, accumulating about 11 hours of total sleep through micro-naps while remaining alert enough to protect their nests. Polar swimmers, deep divers, and high-altitude migrants all reportedly show similarly creative adaptations of when and how they sleep. The diversity of approaches, in researchers' view, suggests that the underlying need is so strong that animals will find arbitrarily strange ways to satisfy it.

A Sea level is widely said to have risen and fallen by more than 100 metres during repeated glacial cycles over the past 800,000 years. The present pulse of rising seas, beginning in the mid-nineteenth century and accelerating substantially in recent decades, is, in researchers' view, different in cause and probably in consequence: it is driven by human emissions rather than orbital cycles, and is occurring against a backdrop of unprecedented coastal infrastructure that previous sea-level changes did not have to contend with. Cities, ports, refineries, power stations, and tens of thousands of kilometres of coastal road, rail, and pipeline now line the very coastlines that earlier rises and falls reshaped without consequence to human society.

B The principal causes of modern sea-level rise are identified as thermal expansion of seawater and the melting of land-based ice. Thermal expansion accounts for roughly one-third of the rise observed over the past three decades. As ocean water warms, it expands; even a modest warming of the upper ocean is widely said to add substantially to global mean sea level. The remaining two-thirds of recent rise comes principally from the melting of mountain glaciers worldwide, and increasingly from the great ice sheets of Greenland and Antarctica. Sea ice melt, despite its visible drama, does not contribute directly to sea-level rise because the ice was already floating. The relative contributions of each source are widely said to have shifted over time, with ice-sheet melt growing as a share over recent decades.

C The pace of rise has, according to many oceanographers, accelerated. Tide-gauge records reportedly show global mean sea level rose by approximately 1.5 millimetres per year through most of the twentieth century. Since 1993, satellite altimetry has documented a rate of roughly 3.4 millimetres per year, with further acceleration observed in the past decade. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change projects that total rise by 2100 will be between roughly 0.3 and 1.0 metres, depending heavily on emissions trajectories, with substantial probability of rise above 1.5 metres under high-emission scenarios that produce significant Antarctic ice loss. The wide spread of projections is widely said to reflect genuine uncertainty about future ice-sheet behaviour rather than any disagreement about thermal expansion or mountain-glacier contributions.

D The consequences for coastal regions are widely described as uneven. Some areas — including subsiding river deltas like the Mekong and Ganges-Brahmaputra — reportedly experience local sea-level rise far exceeding the global average because the land itself is sinking. Other regions, particularly parts of the Scandinavian coast still rebounding from the weight of vanished ice-age glaciers, are said to be seeing relative sea level fall despite the global rise. The result is that the same global emissions reportedly produce dramatically different consequences for different coastal populations, with some of the most densely-populated low-lying deltas among the most exposed.

E The infrastructure response has varied. The Netherlands has invested heavily in its Delta Works flood-defence system and is now adapting these for projected sea levels through the 21st century. New York's Big U project, planned after Hurricane Sandy in 2012, integrates flood walls, deployable barriers, and public-space redesign to protect lower Manhattan. Smaller-scale projects in Venice, Hamburg, and many other cities typically use mobile barriers, raised buildings, and zoning rules to manage flood risk. Island nations such as Kiribati and Tuvalu reportedly face fundamentally different challenges: their highest points are only metres above sea level, and engineering responses are widely said to be limited by both geography and budget. Some Pacific governments are widely said to have begun preparing legal frameworks for permanent population relocation should engineered defences eventually prove inadequate.

F The economic costs of sea-level rise are, in researchers' view, substantial and concentrated. A 2022 analysis by Climate Central estimated that without further adaptation, land currently home to roughly 300 million people will be below the high-tide line by 2050, with the majority of these people in Asia. Insurance markets in coastal regions of the United States and elsewhere are widely said to be showing significant stress, with some private insurers withdrawing from high-risk areas entirely. The economic distribution of climate impacts is, according to many analysts, increasingly recognised as a major political challenge: those who emit least often live in regions most exposed, while those who emit most generally have more resources to adapt.

G Slow-onset processes generally complicate the picture. Sea-level rise is, in many researchers' view, not abrupt for most regions but the cumulative effect, when combined with storm surges, reportedly increasingly produces serious damage. Many low-lying delta cities face a particular danger of compound floods, in which heavy rainfall, river flooding, and storm surges combine to overwhelm flood defences that would each individually be sufficient for the component event. Adaptation planning therefore increasingly considers compound and cascading effects rather than treating sea-level rise as a simple, slow encroachment of water.