This week is the quietest week for U. S. stocks so far in 2021, and Wall Street is wondering what will break that calm.

The five-day average trading volume on US exchanges fell to 9.5 billion shares, the lowest since October, as the S & P 500 rose to an all-time high and stock trading volumes plummeted, according to Bloomberg data. Friday's trading was particularly quiet, with only 8.7 billion shares changing hands, the lowest daily trading volume since Christmas Eve last year.

Over the past 13 months, U. S. stocks have experienced a slump and then the sharpest rebound in nearly a century. Today's calm seems particularly sudden.

In the US, where the epidemic is still severe, the approval of the vaccine in November sparked more optimism in the market, and US stocks appeared to be switching styles. Since then, more than $575 billion has flowed into the market, more than the total inflow in the past 12 years, according to Bank of America Corporation.

Everything changed in April, and U. S. stocks ushered in a period of calm.

Some analysts believe that the calm of US stocks is closely related to several factors: with the relaxation of economic restrictions, the market's enthusiasm for retail stocks has cooled; after Fed officials have spoken in turn, the brief sell-off in the bond market has led to higher yields; in addition, economic data are beginning to support valuations.

However, the market believes that this calm will not last.

Arthur Hogan, chief market strategist at National Securities, said:

We used to drive at 100 miles an hour, but now we are back within the speed limit.

We will see a resurgence of trading volumes and volatility, as this year we will see dramatic changes in economic growth, income growth, inflation, and note the new framework of the Fed.

After the calm, what is waiting for US stocks?

Wall Street believes that the momentum to continue to push up U. S. stocks may have been exhausted. On Friday, the S & P closed at an all-time high of 4128.8, above the year-end average of 4099 for strategists tracked by Bloomberg.

Sceptics point out that the main reasons for caution in investing in US stocks are rising US bond yields, overvalued US stocks and potential tax increases.

Tobias Levkovich, chief US equity strategist at Citigroup Inc, expects the Fed to ease monetary stimulus later this year, while earnings guidance will weaken, adversely affecting the stock market and exacerbating volatility, with a target of 3800 points for the S & P 500 in 2021.

Bokeh Capital Partners's Kim Forrest is more optimistic. She expects the best earnings season since 2018 to kick off, which will revive the stock market.

Profits of S & P 500 companies may have risen 24% in the first quarter, led by automakers, banks and retailers, according to data compiled by Bloomberg.

Forrest believes that:

Unless special circumstances like the COVID-19 epidemic happen again, we are entering the surplus season, and last year's base is very low, I think the first quarter US stock results will be very good, but also enough to inspire investor confidence.