We have collected blood from the dengue patients in sst tube, separate the serum and store it at -20c. Now how long can we store these samples? Is there any risk of decreasing and degradation the virus titre?
I send some of paper that may help to answer your question. The researcher just exam the viability of Influenza virus with various conditions, temperature and time point. This all data can guide you to estimate the storage time for your samples.
For DENV Isolation procedures -80°C is a must. The best would be to do the Isolation from the fresh sample and not freeze the sample. -20°C is fine for RT-PCR