Why Mesoblast (ASX:MSB) and this ASX 200 share have doubled in value in 2020

I think it is fair to say that it has been a year to forget for the S&P/ASX 200 Index (ASX: XJO).

Since the start of the year, the benchmark index has lost 13.5% of its value.

The good news is that not all shares on the index have dropped lower this year. In fact, a couple of shares are not only beating the market, they have more than doubled in value in 2020.

Here's why they are on fire this year:

Afterpay Ltd (ASX: APT)

The Afterpay share price is the best performer on the ASX 200 in 2020 with a whopping 161% gain. Impressively, this is despite the buy now pay later provider's shares trading 20% lower than their 52-week high.

The catalyst for this strong gain has been the rapid growth in customer and underlying sales numbers during the pandemic. This has been driven by an acceleration in the shift to online shopping and the growing popularity of the payment method with consumers and merchants. Also getting investors excited is its expansion plans. Afterpay recently revealed plans to enter the European market and also has its eyes on the Asian market.

Mesoblast limited (ASX: MSB)

The Mesoblast share price is up 141% since the start of the year. Investors have been buying this biotechnology company's shares due to positive developments relating to its remestemcel-L product candidate. Remestemcel-L is being developed as a treatment for paediatric steroid-resistance acute graft versus host disease (paediatric SR-aGvHD).

In August the company had a meeting with the Oncologic Drugs Advisory Committee (ODAC) of the U.S. FDA to discuss remestemcel-L as a potential treatment for paediatric SR-aGvHD. Pleasingly, after some initial doubts, the ODAC was supportive of remestemcel-L and gave it the thumbs up. While this doesn't guarantee FDA approval, it's a huge step forward. In addition to this, there is excitement around the company's trials of remestemcel-L in ventilator-dependent COVID-19 patients with acute respiratory distress syndrome. Phase 3 trials are currently underway in Australia and the United States.