Viral Epidemiology: Questions

Surveillance is often hardest where it is most needed, at the sites where emerging viruses first appear.

• Weak laboratory infrastructure. Limited testing capacity and unreliable supply chains delay or prevent confirmation, and reference testing may be far away.
• Incomplete vital registration. Many deaths go unregistered or uncertified, so mortality data are poor; verbal autopsy is sometimes the only tool.
• Under-reporting. Passive systems capture only a fraction of cases where access to care is low and reporting is not resourced.
• Workforce and funding gaps. Too few trained field epidemiologists and competing priorities limit timely analysis and response.
• Mitigations. Sentinel sites, community-based surveillance, mobile-phone reporting, simple tools and field-epidemiology training programmes extend reach at lower cost.

Surveillance is the ongoing, systematic collection, analysis, interpretation and dissemination of health data, linked to action: “information for action”.

• Systematic and continuous. Data are gathered routinely against a standard case definition, not ad hoc.
• The surveillance cycle. Collection feeds analysis and interpretation, then dissemination, then public-health action, with feedback to the providers of the data; a weak link degrades the whole.
• Fit for purpose. Systems are chosen and judged against attributes that trade off, chiefly sensitivity, timeliness, representativeness and predictive value.
• A clinical trigger plus laboratory confirmation. Effective viral surveillance pairs a syndromic or clinical signal (influenza-like illness, acute flaccid paralysis) with laboratory confirmation and, increasingly, genomic and environmental data.

Prevalence = 150,000 / 1,000,000 = 15%, the proportion of the population infected at this point in time.

Predictive value depends on prevalence, not only on a test’s sensitivity and specificity. As prevalence rises, the positive predictive value (PPV, the chance that a positive result is a true positive) rises, and the negative predictive value (NPV, the chance that a negative result is a true negative) falls.

At low prevalence a larger share of positive results are false positives, even with a highly specific test, because the few true positives are outnumbered by false positives drawn from a large uninfected majority.

A screening test is therefore most trustworthy for confirming disease when applied in a higher-prevalence setting, such as during an outbreak or in a high-risk group. A positive result in a very-low-prevalence setting should be confirmed before acting.

Because neutralising antibody often persists for years or life, seroprevalence measures a population’s cumulative experience of a virus rather than current activity. Reading it by age group shows how efficiently and how recently the virus has circulated.

It does not by itself indicate active infection, ongoing shedding, or guaranteed protection against reinfection.

A case-control study is retrospective, starting from cases and matched controls and comparing their prior exposures. It is efficient for uncommon or slow diseases but is vulnerable to recall and selection bias.

A cohort works forward from exposure to outcome; a cross-sectional study is a single-time-point snapshot; and a trial assigns the exposure deliberately.

An animal reservoir sustains the virus independently of human immunity, so a zoonosis cannot be eradicated by controlling human infection alone. The absence of a reservoir was one of the decisive factors that made smallpox eradicable.

Such infections can still be diagnosed, vaccinated against, and kept under surveillance.

A virus that relies on vertical transmission needs the host to survive and reproduce, so selection pushes it toward lower virulence, since killing or sterilising the host is a reproductive dead end.

Higher virulence is more readily tolerated by horizontally transmitted viruses, which do not depend on the host’s reproductive success to reach a new host.

Virus produced at a site with no route to a body surface reaches a transmission dead end and is not shed, even when the infection there is severe or lethal.

Shedding requires a portal that communicates with the outside, which the brain and most internal organs do not provide.

When the human R0 is near zero the virus cannot sustain human chains, so every case depends on fresh spillover from the reservoir, the pattern seen with Lassa fever in West Africa. Only once R0 approaches and passes one can sustained human transmission occur.

An epidemic is a rise in incidence that exceeds the expected endemic baseline for a population, implying unusually wide and rapid spread.

Multi-continent spread is a pandemic; the steady background level is endemic; and scattered unrelated cases are sporadic.

Varicella-zoster virus establishes latency and reactivates decades later as zoster, reseeding chickenpox in a new generation and bridging the gap between epidemics without needing a large susceptible pool. Measles, an acute infection with lifelong immunity and no reservoir, fades out below its critical community size.

Measles is in fact the more transmissible of the two, neither has an animal reservoir, and chickenpox does not spread by the water-borne route.

At low prevalence the positive predictive value falls, so a larger share of positive results are false. Predictive value depends on prevalence, not only on a test’s sensitivity and specificity, which is why surveillance signals are confirmed before action.

Enveloped respiratory viruses are labile and survive poorly outside the body, so the respiratory route depends on relatively close or repeated contact.

They do not persist on surfaces or in water the way hardy, non-enveloped enteric viruses do.

The herd-immunity threshold is 1 minus 1/R₀, so at an R₀ of 5 about 80% of the population must be immune for each case to produce fewer than one further case.

The more transmissible the virus, the higher the threshold, which is why measles, with an R₀ around 12 to 18, needs roughly 92 to 95%.

For most viruses the site of shedding mirrors the site of entry, so a virus entering by the respiratory tract is generally shed from it, and a gut virus is shed in faeces.

Generalised infections are the exception, shedding from several sites at once, as cytomegalovirus does into saliva, urine, semen and milk.

Both are healthcare-associated, but iatrogenic transmission is spread specifically by a medical procedure or practitioner, the 1976 Ebola outbreak in Zaire, amplified by reused unsterilised needles, being the extreme example.

Nosocomial transmission is the broader category of acquiring a virus in a hospital or clinic.

Comprehensive donor screening has made transfusion transmission rare, so the blood-borne viruses now spread mainly through shared injecting equipment.

They are not transmitted by the respiratory, faecal-oral or casual-contact routes, and unbroken skin is an effective barrier.

Overdispersion means transmission is uneven, with a small number of cases or events causing most secondary infections, the superspreading pattern seen with SARS and SARS-CoV-2.

It makes an epidemic’s early course erratic and means control focused on high-transmission settings can be disproportionately effective.

SARS in 2003 had little pre-symptomatic transmission and a generation time of about a week, so isolating symptomatic cases and quarantining their contacts interrupted spread.

A virus that transmitted mostly before symptoms appeared would defeat the same symptom-based measures.

R₀ is the product of three components: how efficiently the virus transmits at each contact, how many contacts an infected person makes, and how long that person stays infectious.

The other options combine quantities that do not compose the reproduction number.

R₀ is the average number of secondary infections produced by one infected individual in a wholly susceptible population, and its threshold value of 1 separates fade-out from epidemic spread.

The other options describe the immune fraction, the contact rate, the clinical ratio, and the generation time.

After a vector takes an infected blood meal, the extrinsic incubation period is the time the virus multiplies and reaches the salivary glands before the vector can transmit it, and it shortens at higher ambient temperature.

The intrinsic incubation period, by contrast, is the interval that runs in the human host after the bite.

Infectivity is quantified by the dose that infects half of those exposed, the ID50.

Virulence, by contrast, is the probability of severe disease and is read from the case-fatality rate; the reproduction number and shedding duration measure different things again.

Prevalence is a snapshot: the cases present in a population at one moment divided by the population size. It suits chronic or insidious infection, where onset is hard to date.

Incidence and attack rate count new cases over time; the seroconversion rate tracks new antibody responses; and the case-fatality rate is the proportion of cases who die.

Prevalence is a function of both incidence and duration, so with steady new-case rates a rising prevalence reflects longer duration, whether from improved survival or greater chronicity.

Faster transmission or waning immunity would raise incidence; higher case-fatality would shorten duration and lower prevalence; and subclinical infection does not add to the count of recognised cases.

The classical droplet-versus-aerosol cut-off is best read as an approximation, because respiratory spread sits on a continuum rather than splitting cleanly into two mechanisms, a view reinforced by the reappraisal that followed SARS-CoV-2.

Fine aerosols can travel beyond a metre or two and linger in the air, so the boundary is not sharp.

Transovarial transmission passes the virus to the vector’s next generation through the egg, which helps some arboviruses survive between transmission seasons.

It is distinct from replication in the vector, from mechanical carriage on the mouthparts, and from biological transmission to a human host.

Faecal-oral viruses are typically non-enveloped and survive in water and on surfaces for days to weeks, which is what lets them spread through contaminated food, water and hands.

Enveloped viruses, by contrast, are labile in the environment and do not persist well enough to use this route.

When the latent period is shorter than the incubation period, a person becomes infectious before symptoms appear, so transmission is pre-symptomatic, as it is for measles and chickenpox.

This blunts any control measure that relies on detecting symptoms before a person can spread the virus.

The secondary attack rate is the proportion of susceptible contacts of an index case who become infected within one incubation period. It is a useful measure of infectiousness in closed groups such as households.

The whole-population figure is ordinary incidence; deaths among contacts is a mortality measure; point-source totals describe an outbreak’s size; and the antibody-positive fraction is seroprevalence.

A cross-sectional study takes a snapshot at one time point and measures prevalence, quickly and cheaply, but it captures cause and effect together and cannot show which came first.

Case-control and cohort designs track exposure and disease over time; a trial tests an intervention rather than measuring baseline prevalence.

When a virus reaches a population with no prior experience of it, the attack rate approaches 100% and adults suffer disproportionate mortality, largely because so much of the community is ill at once that care and social life collapse, rather than because of any special genetic susceptibility.

The attack rate is high rather than low, and illness is not confined to children.

Genome sequencing, the tool of molecular epidemiology, distinguishes vaccine from wild poliovirus and can track reversion toward virulence. It also assigns outbreak strains to a geographic origin.

Serology, culture, antigen detection and microscopy can identify or quantify the virus but do not resolve strain origin at the sequence level.

Incidence counts new cases over a defined period and is the natural measure for acute infections, where each case is a discrete, short episode.

Prevalence and seroprevalence suit chronic or persistent infection; the case-fatality rate measures severity rather than frequency; and duration is a component of prevalence, not a frequency measure in its own right.

Vertical transmission occurs by one of three mechanisms: integration into the germline, transplacental spread in pregnancy, and perinatal or postnatal spread at delivery and through breast milk.

The other routes listed are all horizontal, passing the virus between individuals in a population.

Vertical transmission passes a virus from parent to offspring, in utero, around birth, or through breast milk, so mother-to-infant HIV at delivery is the vertical example.

The others are all horizontal transmission, between individuals in a population, by the respiratory, faecal-oral, contact and vector routes.

Indirect transmission bridges a gap through an intermediary, so a contaminated fomite such as a door handle is the indirect route.

The others are forms of direct transmission, which need immediate contact between an infected and a susceptible host.

An arbovirus must replicate inside its arthropod vector, completing the extrinsic incubation period from gut to salivary glands. This biological transmission is the defining criterion of the group.

Carriage on the mouthparts is mechanical transmission, which excludes a virus. A DNA genome, faecal-oral spread and a human reservoir are irrelevant to the definition; most arboviruses have RNA genomes and non-human reservoirs.

Pathogenicity is the qualitative capacity of a virus to cause disease at all: a virus either is pathogenic or it is not. Virulence is the quantitative degree of disease it causes, the property used to compare a virulent strain against an attenuated one. Option A reverses the two.

The terms are not synonymous (C), and neither describes transmissibility or infectivity, which are separate properties (D). Virulence is not fixed: it is relative to the host and is meaningfully compared only between related viruses (E).

A prospective cohort follows an exposed population forward and gives the strongest observational evidence of cause and effect, which is why it is essential for defining vaccine safety and efficacy. Its cost is time and expense.

The other designs cannot establish the same forward link between exposure and outcome.

In active surveillance health officials take the initiative to obtain reports, giving high completeness and timeliness, but it is resource-intensive and reserved for priority diseases. Passive surveillance is cheaper and wider but under-reports.

A large share of infections are subclinical, so counting only clinical disease undercounts total infection. The gap can be extreme: under 1% of those infected with poliovirus or an encephalitis-causing arbovirus develop paralysis or encephalitis.

The other options describe reporting delays or immunity effects, not the systematic gap between infection and apparent disease.