Swiss cheese model explains why multiple measures needed to fight Covid-19

Face masks need to be worn correctly, consistently and by enough people, to be effective in helping to prevent viral spread. — SS Kanesan/The Star

There is no single measure that is 100% effective in preventing the spread of the SARS-CoV-2 virus that causes Covid-19.

Measures other than vaccination are needed against Covid-19.

The Swiss cheese model to address the problem of human fallibility in the management of errors was put forward by British psychologist Emeritus Professor James Reason.

Any organisation or system usually has a series of defences, barriers and safeguards against failure.

However, in the real world, these defences are more like slices of Swiss cheese with holes in them.

Unlike the cheese though, these holes are continually opening, shutting and shifting locations, according to a 2000 article he wrote in The BMJ.

Holes in any one slice do not normally cause an adverse outcome.

However, when the holes are momentarily in alignment, hazards can penetrate through all the defensive barriers and come into damaging contact with victims, which in healthcare, would be the patients, he wrote.

Australian virologist Dr Ian Mackay adapted Emeritus Prof Reason’s Swiss cheese model for Covid-19 with his colleagues last year (2020), resulting in the Swiss Cheese Respiratory Virus Pandemic Defence (see graphic).

Additional slices – each representing one of the defences against the virus – were added to Emeritus Prof Reason’s model.

None of the slices are perfect, with each having holes that represent the measure’s flaws, which allow the virus to find a way through.

Yet, with several slices, the likelihood that the virus will be stopped by one of the measures is increased.

The Swiss Cheese Respiratory Virus Pandemic Defence model adapted by Dr Mackay and his colleagues from the Swiss Cheese model of accident causation created by Emeritus Prof Reason. — Dr IAN MACKAYThe Swiss Cheese Respiratory Virus Pandemic Defence model adapted by Dr Mackay and his colleagues from the Swiss Cheese model of accident causation created by Emeritus Prof Reason. — Dr IAN MACKAY

Covid-19 vaccines

Vaccines are generally regarded as one of the most potent measures in addressing Covid-19.

The first real-world effectiveness of the Covid-19 vaccine was reported by Israel, which launched its vaccination programme on Dec 20 (2020), nine days after the US Food and Drug Administration (FDA) authorised the emergency use of the Pfizer/BioNTech vaccine.

All who were vaccinated by the Clalit Institute for Research, which is the country’s largest healthcare provider, from Dec 20 (2020) to Feb 1 (2021) were matched to unvaccinated controls in a 1:1 ratio according to demographic and clinical characteristics, with each group comprising 596,618 persons.

According to their study published in the New England Journal of Medicine on April 12 (2021), two doses of the mRNA (messenger ribonucleic acid) vaccine reduced symptomatic cases by 94%, hospitalisation by 87% and severe Covid-19 by 92%.

The effectiveness after one dose, during days 14 to 20, was 57% for symptomatic cases, 74% for hospitalisation and 62% for severe Covid-19.

The estimated effectiveness in preventing death was 72% in the two to three weeks after the first dose.

However, there was insufficient data to produce an estimate on the reduction in death in those who received two doses.

The US Centers for Disease Control and Prevention (CDC) reported that 75 million people in the United States had been fully vaccinated from Dec 14 (2020) to April 13 (2021).

There were 5,814 vaccine breakthrough infections reported in this cohort (0.00775%), i.e. people who were infected with the SARS-CoV-2 virus despite receiving the vaccine.

Of these, 2,622 (45%) were in people aged 60 years and above, and 3,752 (65%) were female.

A total of 1,695 cases (29%) were asymptomatic (i.e. had no symptoms), 396 people (7%) were hospitalised and 74 (1%) died.

Of the 396 hospitalised patients, 133 (34%) were asymptomatic or hospitalised for a reason not related to Covid-19; and of the 74 fatal cases, 9 (12%) were reported as asymptomatic or the death was not related to Covid-19.

The Health Ministry announced that 40 Malaysian healthcare staff who had completed two doses of the vaccine, developed the infection, but that the symptoms were less severe.

No information was provided about the clinical characteristics of those affected.

In short, although vaccination prevented people from getting sick and dying from Covid-19, there is no vaccine yet that is 100% effective, and there is the remote likelihood that one can get infected or spread the SARS-CoV-2 virus even after vaccination.

Testing and contact tracing

Widespread testing detects people carrying the virus who are unaware they are infected.

The polymerase chain reaction (PCR) test is time-consuming, expensive, and requires specific equipment and expertise not all laboratories have.

The antigen and recently-introduced saliva tests are rapid tests, but may miss some cases as they are imperfect.

A contact-tracing system tracks down those who have been in close contact with a positive case.

But the system has to be fast and able to reach between 80-100% of the patient’s contacts for it to be effective in preventing spread.

The use of digital technology increases its effectiveness, but alas, much of contact tracing in Malaysia is still done manually.

Quarantine and self-isolation

Most countries require Covid-19 positive people or those suspected to be so, to be quarantined either in a healthcare facility or at home.

Some countries also require travellers to quarantine.

The objective is to avoid spreading the infection to others.

But there are instances of non-compliance by all segments of society.

Outdoor socialising and ventilation

Fresh air reduces the likelihood of infection and the ultraviolet radiation from sunlight can destroy viruses left on surfaces.

But the risk outdoors is not reduced to zero.

The likelihood of viral spread is greater in crowded and enclosed spaces, particularly if they are poorly ventilated.

The greater the airflow, e.g. by opening the windows, the less likely the spread, but the danger is still not zero.

Touch and cough

Frequent handwashing or sanitising, cough etiquette and avoidance of touching the face reduces the likelihood of spread, but the risk of aerosol spread remains.

Face masks

Face masks and coverings can block the spread of viral droplets from coughing, sneezing and speaking.

But the virus also spreads by aerosols.

As such, even the highest-grade masks may still let some virus through.

Different face masks have different filtration efficiencies.

Moreover, the masks need to be worn correctly, used constantly and by sufficient people to make a difference.

Physical distancing

Keeping 2m (6 feet) away from another person reduces the likelihood of viral spread.

However, because the virus also spreads by aerosols, there is still the chance you can get infected even if you are more than 2m away from a positive person.

Science-based policies

The US and Brazil, with more than 550,000 and 370,000 deaths respectively, provide examples of failures to adhere to science-based policies.

In the CNN documentary COVID WAR: The Pandemic Doctors Speak Out, Trump White House coronavirus co-ordinator Deborah Birx said: “The first time we have an excuse.

“There were about 100,000 deaths that came from that original surge.

“All of the rest of them, in my mind, could have been mitigated or decreased substantially.”

Despite surging infections, more than 66,000 deaths alone in March and the Brazilian healthcare system near collapse, Brazilian President Jair Bolsonaro continues to downplay the threat of Covid-19.


It is evident that no slice of the Swiss cheese model provides 100% protection against viral spread.

The cheese’s holes will constantly open, shut and shift location depending on individual behaviour and institutional policies.

Even when there is widespread vaccine coverage, multiple measures will be needed to stop the holes from aligning and letting the virus through.

It is only by using a number of slices, i.e. defensive measures, that there is the best likelihood for protection of individuals, families and friends.

Policies have to be based on scientific evidence with transparent data analysis that can withstand scrutiny.

It is futile to base decisions on wishful thinking and what one wants to happen.

It is vital to remember that the virus does not discriminate.

Dr Milton Lum is a past president of the Federation of Private Medical Practitioners Associations and the Malaysian Medical Association. For more information, email The views expressed do not represent that of organisations that the writer is associated with. The information provided is for educational and communication purposes only and it should not be construed as personal medical advice. Information published in this article is not intended to replace, supplant or augment a consultation with a health professional regarding the reader’s own medical care. The Star disclaims all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.

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