By Helen Petousis Harris 28/11/2019 13


Samoa are experiencing a devastating measles epidemic. It is possible that 2-3% of the population will ultimately be infected by the time it is over. Hopefully the mass immunisation campaign currently under way can mitigate some of this, for many it is too late. (Updated 3 Dec 2019)

The first question many people ask is “why?”

The general answer is simple. Samoa has long term low immunisation coverage. The tables below illustrate this. The top table is the first dose of MMR vaccine and the next is the second dose. These figures are some of the lowest in the world.

Estimates of coverage of MMR dose 1 in Samoa. Unicef and WHO Estimates of Immunisation Coverage 2018 Revision

This low immunisation coverage has left Samoa extremely vulnerable to measles – a lit match to dry tinder and gasoline. With the global resurgence of measles, the risk of an importation to this vulnerable community was almost inevitable.

Why has the coverage been so low?

There is a degree of distrust by the Samoan community in the health system. This is not entirely misplaced. Events last year highlighted some of the issues with the health practices when two infants were administered a muscle relaxant drug when it was used to reconstitute a five-dose vial of MMR vaccine instead of water. Both infants died within minutes of injection. The grave errors that led to this occurring are monumental both in number and in magnitude.

Following this tragic event the immunisation programme was halted for months. The public were not informed that it was not a vaccine that caused the deaths. The anti-vaccine movement thrived, spreading misinformation, and vaccine coverage plummeted further. The Washington Post highlighted some of the behaviour and it is pretty disappointing disgusting. Alison Campbell has discussed some of the claims lies spread by those who prey.

Other factors contributing to the magnitude of this epidemic

There are two tools available to us to control epidemics such as measles. One is employing isolation and quarantine, the other is through rapid mass vaccination. These interventions must occur swiftly. If there is no disease in the community, a single case can be considered an outbreak. Once secondary cases appear it is most definitely an outbreak. Some 200 cases had appeared before Samoa declared a problem. Even though the first cases appeared in August, a mass immunisation campaign has only just begun. These things together have contributed to the magnitude of this epidemic.

Why are so many children dying?

The average mortality rate for measles in low/middle income settings is around 2 per 100 cases. Also, I have read some literature that suggests mortality is higher when cases are clustered close together – such as when immunisation coverage is very low – and it may have something to do with infective viral load. Anyway, the death rate we are seeing is not unexpected for a setting such as Samoa. Here is an interview with one of the Australian doctors on the ground, he explains the sort of complications they are dealing with in intensive care.

It is important to keep in mind the level of health care and access to health care in Samoa is not the same as it is in NZ, also there were a few near deaths in NZ during this last epidemic.

How much worse will the Samoan epidemic get?

Predicting is never easy without a crystal ball. However, past, present, and future epidemics can be modelled using mathematics. The more information/data you have about the disease and the population, the better the model.

To estimate the trajectory of the Samoan epidemic, academics from the University of Auckland’s Department of Engineering Science and Auckland Bioengineering Institute used some techniques they have been developing recently. The model uses the number reported cases from Samoa’s National Emergency Operations Centre releases, and to keep it simple it does not use the age/spatial information (because it would become much harder to estimate). In other words, it’s quite a simple model that doesn’t capture all the complexity of the outbreak.

Based on the information available the model predicts a total of 4500-6500 cases, with almost all the remaining cases occurring in the next month – 95% of these within 2-3 weeks and 99% within a month. With current mortality rates (1.3% of reports), we expect around 70 deaths. This is in the absence of the mass campaign now ramping up.*(UPDATED VERSION BELOW)

With further information the model can be refined, and I will update this post accordingly. However, right now this is the best estimate available.

SEIR model estimating epidemic curve of 2019 measles epidemic in Samoa.
Wu, Maclaren, and Suresh. Department of Engineering Science and Auckland Bioengineering Institute. University of Auckland. 28 Nov 2019.

 

In the more recent figure (Below) an approach which balances a pure data fit and SEIR model fit (and so basically interpolates the earlier data while relying on the SEIR model to extrapolate beyond) has been used. This is because the SEIR model tends to better later on than early on. The earlier plot just relied the SEIR model, hence the small difference in curve shape.
REVISED SEIR model estimating epidemic curve of 2019 measles epidemic in Samoa. Wu, Maclaren, and Suresh. Department of Engineering Science and Auckland Bioengineering Institute. University of Auckland. 3 Dec 2019

Acknowledgements

Thanks to Mr David Wu, Dr Oliver Maclaren, Dr Vinod Suresh (University of Auckland), and Samoa Observer for the collaboration.


13 Responses to “Samoa’s devastating measles epidemic – why and how bad?”

  • Thanks, sobering indeed. I hope they got enough vaccines in by now to change this trajectory.

  • This article is throwing light on the Samoan devastating measles epidemic. What’s happening to the community is disgusting.
    Hope atleast now the immunization campaign will be geared up.

  • Well there has also been a strong misinformation campaign waged by some antivaxxing zealots in Samoa. They peddle “reports” from dubious websites about the “evils” of vaccination and how everything is a money making scam by the medical community. These frothing zealots , guess social media, encourage Samoans not to vaccinate their children and it’s a disgusting situation that’s also contributed to this tragic epidemic.

  • If someone could post this on Tay’s Way on Facebook that would be great. I can’t as she’s already blocked me for posting pro vaccination information. Thanks
    I

  • If someone could post this on Tay’s Way on Facebook that would be great. I can’t as she’s already blocked me for posting pro vaccination information. Thanks

  • Can we still travel to Samoa if we are vaccinated against measles I have a family holiday booked for Christmas?

    • I think Samoa need the tourism! I would go armed with heaps of compassion, many people will be grieving. To be honest I think most of the epidemic will have waned by then.Keep an eye on the travel alerts.

  • How quickly do you get a benefit from vaccination?
    It’s tripling every two weeks.Samoa’s total population is about 200,000. Given past vaccinations and infections, at this rate it will run short of potential victims in about a month. Will vaccinations now make much difference?

    • Hi David
      Vaccine protection begins after about two weeks after administration. Given the predicted trajectory, my sense is probably not in the scheme of things, it should shorten the duration a little and it will prevent cases but in terms of the size of the epidemic…

  • Thanks Helen, for the analysis. It is all make sense low coverage is the biggest contributing factor. Samoa has a lot to learn from this epidermic.

  • Why did NZ’s health authorities not prepare for this outbreak once it had occurred here well before the Samoan outbreak? Any personal temperature tests carried out?Why were aircraft allowed to fly in and out of that country as normal? Have these a/craft air systems been tested? Why didn’t authorities forsee this event occurring with the full knowledge that Samoan immunisation levels were low? (When we were threatened with bird flu we purchased and distributed at National/DHB level all the Tamiflu we could get hold of?)

    • Hi Pete
      These are frustrations many of us share. Both the NZ and Samoan epidemics were predictible, authorities were warned multiple times, including by the WHO. What can i say!