Table 2.

Parameter estimates and goodness of fit

	2016 to 2018		2019
Estimated parameters	Cross-protection	Neutral	Neutral
$R_0^A$	2.54	2.59	3.07
$R_0^B$	2.30	1.56	2.41
${\chi }_{AB}$	0.0061
${\chi }_{BA}$	0.95
$b_1^A$	0.28	0.44	0.31
$b_1^B$	0.27	0.38	0.3
${\rho }_A$	0.0038	0.0062	0.0032
${\rho }_B$	0.0014	0.0023	0.0019
$t_0^A$	0.15	0.13
$t_0^B$	0.18	0.15
logLik	−3,669.89	−3,626.61	−631.54
AIC	7,359.78	7,269.21	1,287.08
$\mathrm{\Delta }$AIC	90.56	0

Cross-protection and neutral models were formally contested to assess relative goodness of fit in the 2016 to 2018 seasons. Goodness of fit was assessed using the Akaike information criterion (AIC), calculated by fitting models to weekly, type-specific, incidence data for influenza for Massachusetts. See Materials and Methods for definitions of parameters and SI Appendix, Table S3 for associated 95% confidence intervals.