Vaccination and fitness for work

Vaccination works on the last link of the chain of infection: it makes people less susceptible. For several infections that matter at work, including hepatitis B, measles, influenza, pertussis and chickenpox, vaccination is the most effective control available. This lesson explains how vaccines protect individuals and groups, which vaccines are commonly recommended for workers, how workplaces run vaccination programs, and how fitness-for-work decisions, sick leave and return to work fit into infection control.

How vaccines protect

A vaccine trains the immune system to recognize an infectious agent without causing the disease. Some vaccines give long-lasting protection after a full series, such as measles and hepatitis B. Others need regular boosters or updating, such as tetanus every ten years and influenza every year, because immunity wanes or the virus changes. No vaccine is 100 per cent effective, but even partial protection reduces the chance of infection, the severity of illness and, for many vaccines, the chance of passing the infection on.

Vaccines also protect people who are not vaccinated or who do not respond well to vaccines, such as infants, people with weakened immune systems and frail older adults. When enough people are immune, an infected person meets too few susceptible people to keep the chain going, and outbreaks fade. This is called community immunity or herd immunity.

Reproduction numbers and the herd immunity threshold

The basic reproduction number, R₀, is the average number of people one infected person infects in a fully susceptible population. Measles has an R₀ of roughly 12 to 18, among the highest of any disease; seasonal influenza is often around 1 to 2. The effective reproduction number, Rₜ, is the average number actually infected at a given time, which falls as more people become immune. An outbreak grows when Rₜ is above 1 and shrinks when it is below 1.

If a fraction p of the population is immune, then in a simple model Rₜ = R₀ × (1 − p). Setting Rₜ below 1 gives the herd immunity threshold:

# Herd immunity threshold for different basic reproduction numbers (simple model)
for disease, r0 in [("seasonal influenza (approx.)", 1.5), ("an R0 of 3", 3.0),
                    ("pertussis (approx.)", 12.0), ("measles (approx.)", 15.0)]:
    pc = 1 - 1 / r0
    print(f"{disease:<28} R0 = {r0:>4}: threshold {pc:.0%} immune")

seasonal influenza (approx.) R0 =  1.5: threshold 33% immune
an R0 of 3                   R0 =  3.0: threshold 67% immune
pertussis (approx.)          R0 = 12.0: threshold 92% immune
measles (approx.)            R0 = 15.0: threshold 93% immune

For measles, about 93 to 95 per cent of people need to be immune to stop sustained spread. That is why communities with lower coverage have outbreaks even when national coverage looks high, and why Canada lost its measles elimination status in November 2025. Real populations are not evenly mixed, so pockets of low coverage in a school, congregation or workplace can sustain spread even when the overall rate is above the threshold.

Simulating an outbreak with an SIR model

Epidemiologists use compartment models to explore how infections spread. The simplest, the SIR model, divides a population into Susceptible, Infectious and Recovered (immune) groups. Each day, susceptible people become infected at a rate proportional to contacts with infectious people, and infectious people recover at a fixed rate. Starting with part of the population already immune by vaccination shows how coverage changes the size of an outbreak.

# Deterministic SIR model (Euler steps) of a 500-person workplace and its contacts.
def outbreak(r0=3.0, infectious_days=5.0, vaccinated=0.0, n=500, days=200):
    gamma = 1 / infectious_days
    beta = r0 * gamma
    s = n * (1 - vaccinated) - 1
    i, r = 1.0, n * vaccinated
    peak = 0.0
    for _ in range(days * 10):            # time step 0.1 day
        new_inf = beta * s * i / n * 0.1
        new_rec = gamma * i * 0.1
        s, i, r = s - new_inf, i + new_inf - new_rec, r + new_rec
        peak = max(peak, i)
    total = n - s - n * vaccinated        # people infected during the outbreak
    return total, peak

for v in (0.0, 0.3, 0.5, 0.6, 0.7):
    total, peak = outbreak(vaccinated=v)
    print(f"{v:.0%} vaccinated: about {total:5.0f} infected, peak {peak:4.0f} ill at once")

0% vaccinated: about   471 infected, peak  152 ill at once
30% vaccinated: about   288 infected, peak   60 ill at once
50% vaccinated: about   147 infected, peak   16 ill at once
60% vaccinated: about    66 infected, peak    4 ill at once
70% vaccinated: about     8 infected, peak    1 ill at once

With an illustrative R₀ of 3, the model's herd immunity threshold is about 67 per cent. Below it, the outbreak still infects many people, though fewer as coverage rises; above it, the outbreak fizzles out after a handful of cases. The model is simplified: it assumes a perfect vaccine, even mixing and no other controls. In reality, other layers such as ventilation, staying home when sick and masking lower the effective reproduction number too, which is why layered controls work even when vaccine coverage is incomplete.

Vaccines commonly recommended for workers

Vaccine recommendations for adults in Canada come from the National Advisory Committee on Immunization (NACI) through the Canadian Immunization Guide, with provinces running their own programs. For workers, recommendations depend on the job.

Running a workplace vaccination program

Employers in health care and other higher-risk sectors often run vaccination programs, and in some cases the law requires them to offer specific vaccines, as with hepatitis B for workers covered by bloodborne pathogen rules. A good program:

• identifies which jobs need which vaccines through the hazard assessment;
• offers recommended vaccines at no cost and during working hours where possible;
• keeps confidential records of vaccination and immunity, separate from personnel files, and of informed declinations;
• provides clear information about benefits, risks and side effects;
• reviews records before staff start in roles where proof of immunity is expected, such as measles immunity in health care;
• has a plan for staff who cannot be vaccinated or decline, such as reassignment during outbreaks;
• runs annual campaigns for seasonal vaccines, with easy access such as on-site clinics.

Mandatory vaccination policies are legal in some settings but must be reasonable and must accommodate people who cannot be vaccinated for medical reasons or other grounds protected by human rights law, up to the point of undue hardship. Courts and arbitrators in Canada have upheld some health care vaccination requirements and struck down others, depending on the evidence, the setting and the alternatives available. Some provinces have rules for particular sectors, such as long-term care. Get advice before introducing a mandatory policy, and involve the joint committee.

Fitness for work and staying home when sick

Staying home when infectious is one of the most effective controls, and also one of the hardest to achieve, because many workers cannot afford to lose pay or fear letting colleagues down. A good policy sets out which symptoms mean staying home, how long to stay away, how to report absence, whether remote work is an option and what pay or leave applies. Common exclusion rules in health care, child care and food service include staying away until 48 hours after vomiting or diarrhea stops, and until fever has gone without fever-reducing medication for at least 24 hours with improving symptoms for respiratory illness. Public health guidance sets the details for specific diseases and settings.

Sick leave law varies by province. Federally regulated employees are entitled to ten days of paid medical leave a year under the Canada Labour Code, in effect since December 2022. British Columbia requires five paid sick days a year, and Quebec gives two paid days among the days of absence allowed for illness or family obligations. Ontario's Employment Standards Act gives three unpaid, job-protected sick days a year, and since October 2024 employers cannot require a note from a doctor, nurse practitioner or other qualified health practitioner for those days, although they may ask for other reasonable evidence. Other provinces have their own rules. Employers can offer more generous leave, and many do because it reduces spread.

# Cost of influenza-like illness in a 200-person workplace (illustrative)
staff = 200
attack_rate = 0.10            # share who get influenza-like illness in a season
days_off = 3.0                # average days missed per case
presenteeism_days = 2.0       # days worked while ill at half productivity
daily_value = 350             # value of a worker-day, dollars
cases = staff * attack_rate
absence = cases * days_off * daily_value
present = cases * presenteeism_days * 0.5 * daily_value
print(f"cases: {cases:.0f}")
print(f"absence cost: ${absence:,.0f}")
print(f"presenteeism cost: ${present:,.0f}")
for coverage, effectiveness in [(0.4, 0.5), (0.7, 0.5)]:
    avoided = cases * coverage * effectiveness
    print(f"{coverage:.0%} vaccinated, {effectiveness:.0%} effective: about {avoided:.0f} cases avoided")

cases: 20
absence cost: $21,000
presenteeism cost: $7,000
40% vaccinated, 50% effective: about 4 cases avoided
70% vaccinated, 50% effective: about 7 cases avoided

Return to work after illness or exposure

Return-to-work decisions balance the worker's health, the risk to others and the job. For most short illnesses, workers return when they meet the exclusion criteria. After certain exposures, such as a non-immune health care worker exposed to measles or chickenpox, public health may require the worker to stay away during the incubation period even if they feel well. Workers with long-term infections, such as hepatitis B or HIV, can usually work normally; restrictions apply only to a small number of procedures in health care where there is a risk of transmission to patients, and decisions are made case by case by expert panels with strict confidentiality. Discrimination against workers because of an infection that poses no real risk is prohibited by human rights and disability law.