update to paper links in README

Author: nickreich

README.md

@@ -8,7 +8,8 @@ There is a lot of data and code in this repository. We have over 20 models' fore
 
  - This collaboration is currently centered around participation in the [CDC FluSight challenge](https://predict.phiresearchlab.org/) which has run since the 2013/2014 season.
  - In the fall of 2017, [Nick](http://reichlab.io) wrote a [blog post describing the new FluSight Network effort effort](http://reichlab.io/2017/11/28/flusight-ensemble.html). This is a good place to start to get an overview of this collaborative effort.
- - A [pre-print of the first academic paper resulting from this collaboration is available on biorxiv](https://www.biorxiv.org/content/early/2018/08/24/397190). This paper compares performance of the models over the 2010/2011 through 2016/2017 flu seasons. The [source code for manuscript itself](https://github.com/FluSightNetwork/cdc-flusight-ensemble/tree/master/writing/comparison) also lives in this repository.
+ - [The first academic paper resulting from this collaboration was published in January 2019 in PNAS](https://www.pnas.org/content/early/2019/01/14/1812594116). This paper compares performance of the models over the 2010/2011 through 2016/2017 flu seasons. The [source code for manuscript itself](https://github.com/FluSightNetwork/cdc-flusight-ensemble/tree/master/writing/comparison) also lives in this repository.
+ - [The second academic paper resulting from this collaboration is available as a pre-print.](https://www.biorxiv.org/content/10.1101/566604v1) This paper describes our work to build a collaborative multi-model ensemble in the 2017/2018 influenza season. 
  - We have an interactive visualization of the forecasts available at [flusightnetwork.io](http://flusightnetwork.io).
  - The technical details and structure of this repository is described on [the wiki for this repository](https://github.com/FluSightNetwork/cdc-flusight-ensemble/wiki).
  - We have [specific guidelines for how to join the FluSight Network and participate](https://github.com/FluSightNetwork/cdc-flusight-ensemble/blob/master/guidelines.md) in the current season.