Epidemiology and population health Flashcards
Describe the basic reproduction rate, R0
R0 = the average number of persons infected by one disease source.
R0 = C x P xD
C: the number of contacts of an infected person makes per unit of time
P: probability of transmission per contact
D: the duration the infected person is infective to others.
If the R0 > 1 the disease will continue to spread.
What does the infection cycle need?
source, host, and spread – amplification.
What is incubation period?
time between catching an infection and symptoms appearing. Varies between different diseases, e.g. rubella (14-21 days).
What is infectious period?
the time period during which an infected person is able to transmit the disease to a susceptible host or vector. Not necessarily associated with symptoms (chicken pox is infectious 1-2 days before symptoms appear.
What is latent period?
time between catching an infection and diagnostic signs of infection but still asymptomatic → when they are capable of infecting others
Define epidemic
affecting or tending to affect an atypically large number of individuals within a population, community, or region at the same time
Define outbreak
a sudden rise in the incidence of disease
Define pandemic
occurring over a wide geographic area and affecting an exceptionally high proportion of the population
Define attack rate
In epidemiology, an attack rate is the biostatistical measure of frequency of morbidity, or speed of spread, in an at risk population. An at risk population is defined as one that has no immunity to the attacking pathogen which can be either a novel or established pathogen. It is used to project the number of victims to expect during an epidemic.
Define secondary attack rate
the secondary attack rate measures the spread of disease within a household, or similarly limited situation. For example a specific rate could be used to evaluate the cause-specific mortality rate due to HIV for a particular group during a particular time.
What is a case report?
study of one individual with a disease, timely or rare information.
What is a case series?
study of several patients with similar symptoms. May lead to general hypothesis. No controls
What is an ecological trial?
at least one variable is measured at group level (e.g. sunlight exposure). The incidence of disease is compared in different groups. The unit of analysis is a population, not an individual. No info on each individual.
Usually used to measure prevalence and incidence of disease, particularly when disease is rare
What is a cross-sectional study?
measures exposure and disease in a study group at one time point (‘’snapshot’’). Frequently takes the form of a survey. Relatively inexpensive, studies often rely on data collected for other purposes.
- e.g. John Snow and cholera in London
What is a case-control study?
assesses whether historical exposure to one or more risk factors in people who have the disease (‘’cases’’) is comparable to that in people who do not have the disease (‘’controls’’). Retrospective study.
Process of case control study
Choose a well-recorded population containing some people who have or did have a precisely defined disease.
Hypothesise what may have caused illness or death
Select matched population who might have developed the pathology but didn’t
Looks back to compare how frequently the exposure to a risk factor is present in each group to determine the relationship between the risk factor and the disease
Compare the answers from the two groups.
Positives of case control studies
Good for rare disease
Relatively fast
Relatively inexpensive
Can look at the association between the disease of interest and many kinds of exposures.
Negatives of case control studies
Susceptible to bias
May be hard to find suitable controls
Time relations may not be clear
Associations that are revealed may not be causal.
What is a randomised control trial?
assess whether a cause-effect relationship exists between treatment and outcome.
What are the phases of clinical trials?
Phase 0: small number of participants and low dose of drug. Confirmation that drug behaves as expected from the laboratory research.
Phase 1: small trials, cohorts are treated with increasing doses of drug (dose escalation study). Safe dose? Side effects? Effect on the disease?
Phase 2: more patients involved. Best dose? How to manage side effects?
Phase 3: New treatment is compared to standard treatment. May involve thousands of patients because differences between treatments may be quite small.
Phase 4: after licencing the long term risks and benefits are studied on very large groups of people.
What is a cohort study?
subjects are divided into two groups, generally those who have been exposed to a risk factor and those who have not.
What is a risk factor?
actor which is identifiable as being statistically significantly associated with a disease but has not yet been proven to be an integral part of the disease mechanism
TB risk factors
Co-infection with HIV, and other forms of immunosuppression (such as undergoing chemotherapy) are major risk factors.
High levels of smoking
Silicosis: silica particles irritate the respiratory system , causing immunogenic responses such as phagocytosis which results in high lymphatic vessel deposits; it is probably this interference and blockage of macrophage function that increases the risk of TB.
Low body weight
Diabetes mellitus: have a poorer response to treatment due to impaired drug absorption
Crowding: transmitted by droplet infection
Risk factors for HIV
Unprotected sex
Poorly sanitised health care equipment
Risk factors for malaria
Living in a tropical area where mosquito is common
Young children and infants
Previously being unexposed to the disease
Pregnancy
Poverty
Describe MRSA
bacterial infection resistant to a number of widely used antibiotics; methicillin resistant staphylococcus aureus. The overuse of antibiotics has played a major role in antibiotic resistance and superbugs, this includes the use of antibiotics to treat minor conditions that would have got better anyway and not finishing recommended courses of antibiotics.
Describe C.dif
clostridium dificile normally affect hospital inpatients due to use of antibiotics and their effects on the normal flora of the gut. Lives on surfaces for many months, and spread via faeces.
Describe norovirus
genus of genetically diverse single-stranded RNA, non-enveloped viruses in the Caliciviridae family. Outbreaks of norovirus in public places such as hospitals, nursing homes and schools, are common because the virus can survive for several days on surfaces or objects touched by an infected person.
What is nosocomial?
hospital acquired infections; are infections whose development is favoured by a hospital environment.
What is iatrogenic?
preventable illness or harm resulting from medical treatment or advice to patients.
History of vaccine development
Jenner – small pox
Sabin (attenuated - IgA ad IgG - prevented gut infection too - could not be spread) and Salk (inactivated - IgG) - Polio
What is vaccine efficacy?
defined as the reduction in the incidence of a disease among people who had received a vaccine compared to the incidence among unvaccinated individuals.
Usually measured in a randomised control trial.
Leading causes of infant and childhood mortality and morbidity in resource limited healthcare systems
- vaccine preventable diseases
- Infection due to resistant pathogens - e.g. AMR in streptococcus, pneumonia, malaria
What is surveillence?
continued watchfulness over the distribution and trends of incidence through the systematic collection, consolidation and evaluation of morbidity and mortality reports and other relevant data, and the regular dissemination of data to all who need know.
What is prophylaxis?
refers to treatments given, or actions taken to prevent disease.
What is herd immunity?
a population shows herd immunity if the frequency of immune, non-susceptible people is high enough to break the train of transmission.
Herd immunity can protect the vulnerable: live vaccines can’t be given to immunosuppressed people – before the MMR programme, half the deaths in the UK were in children with leukaemia (chemotherapy is an immunosuppressant).
Neonates are susceptible to serious infection before they get vaccinated since the maternal antibodies are too weak for protection.
To make heard immunity effective both men and women must be vaccinated.
What is critical vaccination percentage?
the percentage of the population that requires immunization before herd immunity can be achieved.
Discuss vaccination coverage
vaccination acts by preventing people joining the infectious pool. The critical level of immunisation to eradicate the disease, Pc, is found by calculating how far the proportion of susceptibles has to be reduced in the face of an agent of a given Reproductive Rate, R0. The higher the R0, the higher Pc needs to be. If an infection is less infectious then the proportion of the population requiring vaccinated to achieve hear immunity is lower.
UK vaccines
DTaP/IPV/Hip (5 in 1): diphtheria, tetanus, whooping cough (pertussis), polio, haemophilus influenza type b.
MenC: meningitis C
Pneumococcal PCV vaccine
Rotavirus: common cause of diarrhoea and sickness.
MMR: measles, mumps, and rubella
HPV: human papilloma virus, cervical cancer.
Vaccination side effects
Vaccination ethics: vaccines are given to healthy people, not people actually suffering from disease, and side-effects therefore cause disease in previously healthy children.
Not uncommon: mild fever, soreness, slight swelling at injection site, myalgia, headache.
Infrequent: lymphadenopathy, short-lived febrile convulsion.
Very rare: anaphylaxis/allergic reaction, Guillain-Barre syndrome, encephalitis.
How may you determine effectiveness of a vaccine?
Randomised clinical trial -most rigorous way of determining whether a cause-effect relation exists between treatment and outcome and for assessing the cost effectiveness for a treatment.
Important features of a randomised clinical trial
Random allocation of intervention groups
Patients and triallists should remain unaware of which treatment was given until the study is complete – although double blind trials are not always appropriate or feasible.
All intervention groups are treated identically, except the intervention group
Patients are normally analysed within the group to which they are allocated, regardless of whether they experienced the intended intervention
The analysis is focused on estimating the size of the difference in predefined outcomes between intervention groups.
Control of reservoirs, vectors
Sanitation is key
Good hand washing procedures
Cleaning of surfaces effectively
Define continuous variable
Any value
Discrete variable
Contained within a finite set of values
Dependent variable
presents the output or effect, or is tested to see if it is the effect.
Independent variable
represent the inputs or causes, or are tested to see if they are the cause.
Confidence interval
is a type of interval estimate of a population parameter and is used to indicate the reliability of an estimate. It is an observed interval (i.e. it is calculated from the observations), in principle different from sample to sample, that frequently includes the parameter of interest if the experiment is repeated.
Odds ratio
is one of three main ways to quantify how strongly the presence or absence of property A is associated with the presence or absence of property B in a given population. If each individual in a population either does or does not have a property “A”, (e.g. “high blood pressure”), and also either does or does not have a property “B” (e.g. “moderate alcohol consumption”) where both properties are appropriately defined, then a ratio can be formed which quantitatively describes the association between the presence/absence of “A” (high blood pressure) and the presence/absence of “B” (moderate alcohol consumption) for individuals in the population.
Z test
A statistical test used to determine whether two population means are different when the variances are known and the sample size is large. The test statistic is assumed to have a normal distribution and nuisance parameters such as standard deviation should be known in order for an accurate z-test to be performed.
Chi-squared test
any statistical hypothesis test in which the sampling distribution of the test statistic is a chi-squared distribution when the null hypothesis is true. Also considered a chi-squared test is a test in which this is asymptotically true, meaning that the sampling distribution (if the null hypothesis is true) can be made to approximate a chi-squared distribution as closely as desired by making the sample size large enough.
