No inicio da pandemia convidaram-me para integrar um daqueles grupos que fazem modelos Covid para o governo Portugues. Eu disse que nao porque queria testar um novo conceito de modelos e queria estar a vontade para fazer ciencia pura e comunicar a vontade.
Comecei a ser contatada por jornalistas que me faziam perguntas as quais eu respondia avisando sempre que os meus modelos eram diferentes dos clássicos.
Talvez nao seja surpreendente que os modelos demorem a ser aceites na especialidade embora eu tivesse alguma esperança que neste caso fosse mais rápido. Mas enfim, abordagens novas demoram a ser processadas.
O que nao estava a contar e que a sociedade inteira começasse a opinar sobre a validade dos meus modelos e o meu caracter. Eu que em geral nao opino sobre questões de sociedade (nem sequer voto porque nao tenho opinião) tive que me envolver e começar a executar um plano de saída.
Ainda nao consegui sair totalmente desta situação na qual me sinto uma personagem que nao eu mas acredito que esteja para breve. Antes de sair deixarei provas da validade dos modelos. Depois desejarei felicidades a todos porque nao sou de ir acertar contas com quem me fez mal.
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1/ Many times I have been asked why communication around herd immunity threshold (HIT) was so confusing in this pandemic. I have even been asked whether experts really understand it. Here is my answer:
2/ The concept is well understood among mathematical epidemiologists. In my view what went terribly wrong was the politicised way in which the HIT was used in this pandemic.
3/ The HIT is the percentage of the population that needs to be immune (prior to vaccination immunity was a natural outcome of recovery from infection) before the epidemic peaks and subsides.
Last year, prominent modelling groups dismissed our Covid work (known for incorporating individual variation in susceptibility and exposure, and estimating low herd immunity thresholds) by claiming our stylised contact-reduction profile (Rc) wasn't close to government NPIs..
England and Scotland (with our latest stylised Rc):
This week, I had a few moments to spare and implemented the same model and fittings with the stringency index that tracks government response. The results are almost identical...
As we approach endemicity new variants are expected to outcompete others faster. Reason being recovered subpopulation (less immune to novel variants as long as there is some immune escape) grows as we approach endemicity increasing the benefit of novelty.
Illustration from model:
When replacement is by immune escape the second variant (B) outcompetes previous faster than first variant (A) had done:
Tweet que e cada vez menos novidade mas infelizmente ainda nao totalmente esclarecido principalmente em Portugal:
Levando em conta heterogeneidades realistas na susceptibilidade/exposição ao SARS-CoV-2 o limiar da imunidade de grupo para linhagens iniciais tera sido menos de 30%.
Se nao tivessem emergido variantes mais transmissíveis nem vacinas (como se equacionava ha mais de 1 ano) estes limiares teriam sido ultrapassados no inverno passado. Vale o que vale mas cada vez temos mais argumentos a suportar esta afirmação.
Desde então temos vivido num equilíbrio precário entre aumento de transmissibilidade do virus (devido as variantes) e diminuição de transmissibilidade (devido as vacinas) que mantem o R efectivo em torno de 1.
1/7 Has the herd immunity threshold (HIT) been used sensibly in the Covid19 pandemic? No! Why does that matter?
2/7 An epidemic (with several waves) may last longer than expected because: (1) it had high potential to begin with and mitigation prevented it from growing vertically so it grew horizontally (single HIT number thinking); or...
3/7 (2) it didn't have as high potential as one might think but viral evolution, seasonality, waning immunity, population renewal, kept it going (dynamic HIT thinking).
Don't know how common this feeling is among mathematical epidemiologists but as someone who has worked on population dynamics of infection & immunity for 20 years I felt hopeless to see herd immunity threshold (HIT) concept degenerating in front of my eyes during pandemic. Thread
1/n HIT is an abstract concept essential to our work but it was hijacked early in the Covid-19 pandemic and disseminated with all sorts of distortions that prevented the impact of its application by those qualified.
2/n A population invaded by an infectious agent (say a virus) achieves HIT when the sum of the immunities acquired by all its individuals is such that the virus cannot cause another epidemic wave.