  Knowledge about the daily number of new infections of Covid-19 is important
because it is the basis for political decisions resulting in lockdowns and
urgent health care measures. We use Germany as an example to illustrate
shortcomings of official numbers, which are, at least in Germany, disclosed
only with several days of delay and severely underreported on weekends (more
than 40%). These shortcomings outline an urgent need for alternative data
sources. The other widely cited source provided by the Center for Systems
Science and Engineering at Johns Hopkins University (JHU) also deviates for
Germany on average by 79% from the official numbers. We argue that Google
Search and Twitter data should complement official numbers. They predict even
better than the original values from Johns Hopkins University and do so several
days ahead. These two data sources could also be used in parts of the world
where official numbers do not exist or are perceived to be unreliable.
