There's no way to be 100% sure, but the most plausible and widely-accepted theory is that the population density in most of Scandinavia (and other more peripheral parts of Europe) was too low for the plague to spread effectively from person to person.

This answer is in many ways guesswork and speculation, but for a lot of the biological/medical aspects of the Black Death, we have little else to go on. We're not even sure if it was Bubonic plague (Yersina Pestis) and if it was, then it was almost certainly a long-extinct variant mutation.

What we do know, however, from the patchy and unrepresentative figures we have, is that while the Black Death was catastrophic for Europe - most estimates are between 33% and 50% of the population killed in the main 1347-50 outbreak, with some ranging as high as 66% - its effects were not evenly distributed. Urban areas, and especially ports, suffered more, and we have confirmed mortality rates of two-thirds-plus of the population in cities like Genoa, Hamburg and Bremen. Rural areas suffered less, but still heavily, as the accounts of deserted villages in England show. Rural areas which were less densely populated, however - and in the 14th century this often meant relying more on mixed agriculture or pastoral farming - seem to have been hit less hard. As a result, in the British Isles the upland north and west were relatively less affected than the more arable and densely-populated south and east. Scandinavian society was closer to the upland, mixed-farming model, and thus seems to have suffered less.

I should stress a few points here though. Firstly, it's not that the plague didn't reach Scandinavia. It did, and it killed a lot of people. Being affected less in this context means losing a quarter of your population instead of half or more. Secondly, Scandinavia wasn't the only place to come off more lightly. Other marginal or peripheral parts of Europe, such as the 'Celtic Fringe' of the British Isles, the Castilian plateau and (more strangely, because they were less marginal, and, at least in parts, more densely settled) large parts of Central Europe, in Poland, Bohemia and Hungary.

So, the plausible explanation seems to be low population density. It certainly wasn't that there was no trade: in fact, much of the trade to Scandinavia would have been routed through northern German cities such as Hamburg and Lubeck, and we know that they were heavily affected. There was an old theory that Scandinavian cold killed the rats, which were how the plague was spread, and that northern regions were affected less, but this is unlikely for several reasons: a) rats can survive in Scandinavia, b) we now believe that the importance of rats as a plague vector has been greatly overstated and c) Scandinavia was affected by the Black Death, just not as much as other places.

Lastly, just some notes on your terminology. Central Europe is generally used in an East-West sense, not a North-South one. So Central Europe usually refers to German Empire east of the Rhineland, along with the Catholic Eastern European kingdoms of Poland, Bohemia and Hungary. Ironically, much of central Europe seems to have surprisingly low plague mortality rates (see above). It's the highly developed, more urbanised and denser societies of Western Europe (Italy, France, England) which suffered the most. Secondly, Vikings are, strictly speaking, Scandinavian or Nordic raiders which were active from the 9th to the 11th centuries, or more generally all Nordic peoples during this period. By the 1340s however, the last vikings had been dead for at least 200 years.

Finally, if you want more info, and in particular a detailed breakdown of what we know of mortality figures from across Europe, try to get your hands on Ole J Benedictow's The Black Death, 1346-1353: A Complete History. While I don't agree with much of his analysis, he's the best source for figures and primary reading. He's also Norwegian and so tends to give some space to places, like Scandinavia, which other scholars brush aside. Any more questions, feel free to follow up and I'll do the best I can!

This is a pedantic terminology note, but the first major wave of bubonic plague was in the 1340s, and the Viking era was maybe up to 1100ish. (Or the plague of Justinian was bubonic plague too, but that was 500s-700s, and the Viking era started 800ish.) You mean 1340s and on against Scandinavians, I guess?