Thread by @AndersonBrito_

Every year Brazil faces a constant threat: dengue virus

After #Zika virus caused major outbreaks in 2016, cases of #dengue declined in 2017-2018, but then resurged in 2019. In this study we investigated the reasons for such fluctuations 📉📈

Thread👇

🔗 medrxiv.org/content/10.110…

Something 'flattened' the dengue curve in 2017-2018. Every year in Brazil, cases of #dengue rise around March, when conditions are favourable (mosquitoes, rain, temperature). But, for some reason, in those two years, dengue cases were low in all regions of the country. 📉🦠

Infectious diseases are transmitted in a population of susceptible hosts following a rate called Force of Infection (FOI).

@RJOidtman, @guido_camargo and @TAlexPerkins estimated dengue FOI for each region in Brazil, and found viral transmission was indeed quite low in 2017-2018.

Dengue declined in Brazil, and in most countries in the Americas. #Chikungunya cases also declined, but gradually, without resurgence in 2019. Using ecological data, @MaryPetrone10 found favourable conditions for dengue spread in 2018-19.

So, why dengue took a "two-year break"?

Where dengue viruses causing recent outbreaks in Brazil came from?

Using phylogenetics and genomes sequenced by @JosephFauver, @mjlsiconelli, @ChaneyKalinich and @GabrielWallau's team at @fiocruz, we found the outbreaks were caused by local lineages circulating for over a decade

In our study we provide further evidence of an interesting phenomenon: immunity from a previous infection by Zika virus may provide cross-protection against future dengue virus infections. This may explain the decline in dengue cases in the Americas ⛔️🦠
medrxiv.org/content/10.110…

#Herdimmunity: when the number of susceptible individuals in a population is low, viruses may yet be able to replicate and transmit, but they do so at very low rates.

That's what probably happened in Brazil: Zika hit hard, and left behind some level of protection against dengue.

We found that when conditions are unfavourable, dengue virus spread cryptically, almost lying in wait, to finally resurge, when more susceptible hosts are available.

Good reason to keep epidemiological surveillance: to detect viruses before they cause large epidemics.💥🦠👀

The distinct parts of this collaborative research involved members of groups coordinated by @NathanGrubaugh at @YaleSPH; @GabrielWallau and Rafael França at @fiocruz_en; @TAlexPerkins at @NotreDame; and Benedito Fonseca at @USPonline.

Bonus track 💿| The pages below show interactive phylogenetic results obtained using @nextstrain

These extra analyses used sequences of a small region (env) extracted from 1000s of dengue virus genomes 🧬

Dengue 1: nextstrain.org/community/grub…
Dengue 2: nextstrain.org/community/grub…

Enjoyed the #phylogenetics #dataviz?✨

Check out these amazing packages:

🌳 Baltic, by @evogytis (Python)
github.com/evogytis/baltic

🌎 Seraphim, by @sdellicour (R):
evolve.zoo.ox.ac.uk/Evolve/Seraphi…

Thank you both for developing and helping me navigate your great tools