Hybrid Retrieval for COVID-19 Literature: Comparing Rank Fusion and Projection Fusion with Diversity Reranking
arXiv cs.CL / 4/16/2026
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Key Points
- The paper introduces a hybrid COVID-19 literature retrieval system on the TREC-COVID benchmark, combining sparse models (SPLADE), dense models (BGE), and fusion strategies (RRF and projection-based B5) to improve relevance and diversity.
- Rank-level fusion (RRF) delivers the best overall retrieval quality with nDCG@10 of 0.828, outperforming dense-only and sparse-only baselines, while projection fusion (B5) trades off some relevance for stronger latency and diversity metrics.
- The B5 projection fusion variant achieves nDCG@10 of 0.678 but is 33% faster (847 ms vs. 1271 ms) and yields 2.2x higher ILD@10 than RRF, with the largest relative gains on keyword-heavy reformulations.
- Applying diversity-oriented reranking via MMR increases intra-list diversity by about 24% (ILD improvements) but reduces effectiveness by roughly 20–25% in nDCG@10, quantifying the relevance–diversity tradeoff.
- The system is implemented as a deployed Streamlit web application using Pinecone serverless indices and keeps end-to-end latency under a sub-2-second target across multiple query types.

