<PAGE>
SUPPLEMENT DATED NOVEMBER 1, 1999 TO THE CURRENT PROSPECTUS
OF
MFS/SUN LIFE SERIES TRUST (THE "TRUST")
This supplement describes a new series of the Trust -- the Strategic Growth
Series -- and supplements certain information in the Trust's Prospectus dated
May 1, 1999. The caption headings used in this Supplement correspond with the
caption headings used in the Prospectus. Information which is not supplemented
applies equally to the Strategic Growth Series.
I. EXPENSE SUMMARY - BEGINNING ON PAGE 1
Expense Table. This table describes the expenses that you may pay when
you hold shares of the series. These fees and expenses do not take
into account the fees and expenses imposed under the Variable
Contracts through which an investment in the series is made. The table
is supplemented as follows:
Annual Operating Expenses (expenses that are deducted from the series'
assets):
Strategic Growth Series
Management Fee 0.75%
Other Expenses 3.43%
-------
Total Annual Series Operating Expenses 4.18%
Fee Waivers/Expense Reimbursement(1) (3.15)%
-------
Net Expenses(2) 1.03%
- -----------------------
(1) MFS has contractually agreed to bear the series' expenses such that "Other
Expenses" do not exceed 0.25% annually. This contractual arrangement will
continue until at least May 1, 2000, unless modified with the consent of
the board of trustees, which oversees the series.
(2) The series has an expense offset arrangement which reduces its custodian
fee based upon the amount of cash maintained by the series with its
custodian and dividend disbursing agent. The series may enter into other
similar arrangements and directed brokerage arrangements, which would also
have the effect of reducing the series' expenses. The series expenses do
not take into account these expense reductions, and therefore do not
represent the actual expenses of the series. Had these expense reductions
been taken into account, "Net Expenses" would be estimated to be 1.00%.
Example of Expenses. The "Example of Expenses" table is intended to
help you compare the cost of investing in the series with the cost of
investing in other mutual funds.
The example assumes that:
o You invest $10,000 in the series for the time periods indicated
and you redeem your shares at the end of the time periods;
o Your investment has a 5% return each year and dividends and other
distributions are reinvested; and
o The series' operating expenses remain the same, except that the
series' total operating expenses are assumed to be the series'
"Net Expenses" for the first year, and the series' "Total Annual
Series Operating Expenses" for subsequent years (see table above).
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The table is supplemented as follows:
Although your actual costs may be higher or lower, under these
assumptions your costs would be:
Year 1 Year 3
Strategic Growth Series $102 $979
II. RISK RETURN SUMMARY - BEGINNING ON PAGE 4
This section of the prospectus is supplemented as follows:
27: Strategic Growth Series
Investment Objective. The series' investment objective is capital
appreciation. Approval by the series' shareholders is not required to
modify or change the series' objective.
Principal Investment Strategies. The series invests, under normal
market conditions, at least 65% of its total assets in common stocks
and related securities, such as preferred stocks, bonds, warrants, or
rights convertible into stock and depositary receipts for these
securities, of companies which MFS believes offer superior prospects
for growth.
MFS uses a bottom-up, as opposed to a top-down, investment style in
managing the equity-oriented funds (such as the series) it advises.
This means that securities are selected based upon fundamental
analysis (such as an analysis of earnings, cash flows, competitive
position and management's abilities) performed by the series'
portfolio manager and MFS' large group of equity research analysts.
In managing the series, MFS seeks to purchase securities of companies
which MFS considers well-run and poised for growth. MFS looks
particularly for companies which demonstrate:
o a strong franchise, strong cash flows and a recurring revenue
stream
o a solid industry position, where there is
potential for high profit margins
substantial barriers to new entry in the industry
o a strong management team with a clearly defined strategy
o a catalyst which may accelerate growth
Consistent with the series' principal investment strategies described
above, the series may invest in foreign securities, and may have
exposure to foreign currencies through its investment in these
securities.
Principal Risks of an Investment. The principal risks of investing in
the series and the circumstances reasonably likely to cause the value
of your investment in the series to decline are described below. The
share price of the series generally changes daily based on market
conditions and other factors. Please note that there are many
circumstances which could cause the value of your
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investment in the series to decline, and which could prevent the
series from achieving its objective, that are not described here. The
principal risks of investing in the series are:
o Market Risk: This is the risk that the price of a security held
by the series will fall due to changing economic, political or
market conditions, or due to the financial condition of the
company which issued the security.
o Growth Companies: This is the risk that the prices of growth
company securities held by the series, which are the series'
principal investment focus, will fall to a greater extent than
the overall equity markets (e.g., as represented by the Standard
and Poor's Composite 500 Index) due to changing economic,
political or market conditions.
o Foreign Securities: Investments in foreign securities involve
risks relating to political, social and economic developments
abroad, as well as risks resulting from the differences between
the regulations to which U.S. and foreign issuers and markets are
subject:
These risks may include the seizure by the government of
company assets, excessive taxation, withholding taxes on
dividends and interest, limitations on the use or transfer
of portfolio assets, and political or social instability.
Enforcing legal rights may be difficult, costly and slow in
foreign countries, and there may be special problems
enforcing claims against foreign governments.
Foreign companies may not be subject to accounting standards
or governmental supervision comparable to U.S. companies,
and there may be less public information about their
operations.
Foreign markets may be less liquid and more volatile than
U.S. markets.
Foreign securities often trade in currencies other than the
U.S. dollar, and the series may directly hold foreign
currencies and purchase and sell foreign currencies through
forward exchange contracts. Changes in currency exchange
rates will affect the series' net asset value, the value of
dividends and interest earned, and gains and losses realized
on the sale of securities. An increase in the strength of
the U.S. dollar relative to these other currencies may cause
the value of the series to decline. Certain foreign
currencies may be particularly volatile, and foreign
governments may intervene in the currency markets, causing a
decline in value or liquidity in the series' foreign
currency holdings. Forward foreign currency exchange
contracts involve the risk that the party with which the
series enters the contract may fail to perform its
obligations to the series.
Bar Chart and Performance Table. The bar chart and performance table
are not included because the series did not have a full calendar year
of performance on December 31, 1998.
III. CERTAIN INVESTMENT STRATEGIES AND RISKS - BEGINNING ON PAGE 76
The series may invest in various types of securities and engage in various
investment techniques and practices which are not the principal focus of
the series and therefore are not described in this Supplement. The types of
securities and investment techniques and practices in which the series may
engage are identified in Appendix A to this Supplement, and are discussed,
together with their risks, in the series' Statement of Additional
Information (referred to as the SAI), which you may obtain by contacting
Sun Life Assurance Company of Canada (U.S.) Retirement Products and
Services Division (see the back cover of the prospectus for the address and
phone number).
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IV. MANAGEMENT OF THE SERIES - BEGINNING ON PAGE 77
Investment Adviser. Massachusetts Financial Services Company is the
series' investment adviser, and is described in the prospectus.
Portfolio Managers.
This section is supplemented as follows:
Series Portfolio Managers
Strategic Growth Series S. Irfan Ali, Vice President of MFS,
and Stephen Pesek, Senior Vice
President of MFS, have been
portfolio managers of the series
since its inception. Mr. Ali has
been employed by MFS as a portfolio
manager since 1993 and Mr. Pesek
has been employed by MFS as a
portfolio manager since 1994.
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APPENDIX A
Investment Techniques and Practices. In pursuing its investment objective,
the series may engage in the following principal and non-principal
investment techniques and practices. Investment techniques and practices
which are the principal focus of the series are described, together with
their risks, in the Risk Return Summary of this Supplement. Both principal
and non-principal investment techniques and practices are described,
together with their risks, in the SAI.
Investment Techniques/Practices
<TABLE>
<CAPTION>
Symbols |X| Permitted -- Not Permitted
Strategic Growth Series
<S> <C>
Debt Securities
Asset-Backed Securities
Collateralized Mortgage Obligations and Multiclass --
Pass-Through Securities
Corporate Asset-Backed Securities --
Mortgage Pass-Through Securities --
Stripped Mortgage-Backed Securities --
Corporate Securities |X|
Loans and Other Direct Indebtedness --
Lower Rated Bonds --
Municipal Bonds --
Speculative Bonds --
U.S. Government Securities |X|
Variable and Floating Rate Obligations --
Zero Coupon Bonds, Deferred Interest Bonds and PIK Bonds
|X|
Equity Securities |X|
Foreign Securities Exposure
Brady Bonds --
Depositary Receipts |X|
Dollar-Denominated Foreign Debt Securities |X|
Emerging Markets |X|
Foreign Securities |X|
Forward Contracts |X|
Futures Contracts |X|
Indexed Securities/Structured Products |X|
Inverse Floating Rate Obligations --
Investment in Other Investment Companies
Open-End Funds |X|
Closed-End Funds |X|
Laddering --
Lending of Portfolio Securities |X|
Leveraging Transactions
Bank Borrowings |X|
Mortgage "Dollar-Roll" Transactions --
Reverse Repurchase Agreements |X|
Options
Options on Foreign Currencies |X|
Options on Futures Contracts |X|
Options on Securities |X|
Options on Stock Indices |X|
Reset Options |X|
"Yield Curve" Options |X|
</TABLE>
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<TABLE>
<CAPTION>
Symbols |X| Permitted -- Not Permitted
Strategic Growth Series
<S> <C>
Repurchase Agreements |X|
Restricted Securities |X|
Short Sales |X|
Short Sales Against the Box |X|
Short Term Instruments |X|
Swaps and Related Derivative Instruments |X|
Temporary Borrowings |X|
Temporary Defensive Positions |X|
Warrants |X|
"When-issued" Securities |X|
</TABLE>
The date of this Supplement is November 1, 1999.
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<PAGE>
SUPPLEMENT DATED NOVEMBER 1, 1999 TO THE CURRENT
STATEMENT OF ADDITIONAL INFORMATION
OF
MFS/SUN LIFE SERIES TRUST (THE "TRUST")
This Supplement describes a new series of the Trust -- the Strategic Growth
Series -- and supplements certain information in the Trust's Statement of
Additional Information dated May 1, 1999. The caption headings used in this
Supplement correspond with the caption headings used in the Statement of
Additional Information. Information which is not supplemented applies equally to
the Strategic Growth Series.
1. DEFINITIONS - PAGE 3
This section is supplemented as follows:
The number "27" replaces the number "26" at the end of the first paragraph.
The following item is added at the end of the list of investment options:
"27. Strategic Growth Series"
2. INVESTMENT TECHNIQUES, PRACTICES AND RISKS - BEGINNING ON PAGE 3
This section is supplemented by adding the following disclosure at the end
of the section on page 5:
"25. Strategic Growth Series
Foreign Securities:................... up to (but not including) 20%
Securities Lending:................... 30%"
3. INVESTMENT RESTRICTIONS - BEGINNING ON PAGE 5
This section is supplemented by adding the following disclosure at the end
of the section on page 11:
"(15) Investment Restrictions that Apply Only to the Strategic Growth
Series:
The Strategic Growth Series may not:
(1) Borrow amounts from banks in excess of 33 1/3% of its assets, including
amounts borrowed.
(2) Underwrite securities issued by other persons except insofar as the Series
may technically be deemed an underwriter under the Securities Act of 1933,
as amended (the "1933 Act") in selling a portfolio security.
(3) Purchase or sell real estate (including limited partnership interests but
excluding securities secured by real estate or interests therein and
securities of companies, such as real estate investment trusts, which deal
in real estate or interests therein), interests in oil, gas or mineral
leases, commodities or commodity contracts (excluding currencies and any
type of option, Futures Contracts and Forward Contracts) in the ordinary
course of its business. The Series reserves the freedom of action to hold
and to sell real estate, mineral leases, commodities or commodity contracts
(including currencies and any type of option, Futures Contracts and Forward
Contracts) acquired as a result of the ownership of securities.
(4) Issue any senior securities except as permitted by the 1940 Act. For
purposes of this restriction, collateral arrangements with respect to any
type of swap, option, Forward Contracts and Futures
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Contracts and collateral arrangements with respect to initial and variation
margin are not deemed to be the issuance of a senior security.
(5) Make loans to other persons. For these purposes, the purchase of commercial
paper, the purchase of a portion or all of an issue of debt securities, the
lending of portfolio securities, or the investment of the Series' assets in
repurchase agreements, shall not be considered the making of a loan. or
(6) Purchase any securities of an issuer of a particular industry, if as a
result, 25% or more of its gross assets would be invested in securities of
issuers whose principal business activities are in the same industry
(except there is no limitation with respect to obligations issued or
guaranteed by the U.S. Government or its agencies and instrumentalities and
repurchase agreements collateralized by such obligations).
In addition, the Series has adopted the following nonfundamental policies
which may be changed by the vote of the Series Fund's Board of Trustees without
shareholder approval. The Strategic Growth Series will not:
(1) Invest in illiquid investments, including securities subject to legal or
contractual restrictions on resale or for which there is no readily
available market (e.g., trading in the security is suspended, or, in the
case of unlisted securities, where no market exists), if more than 15% of
the Series' assets (taken at market value) would be invested in such
securities. Repurchase agreements maturing in more than seven days will be
deemed to be illiquid for purposes of the Series' limitation on investment
in illiquid securities. Securities that are not registered under the 1933
Act and sold in reliance on Rule 144A thereunder, but are determined to be
liquid by the Series Fund's Board of Trustees (or its delegee), will not be
subject to this 15% limitation.
(2) Pledge, mortgage or hypothecate in excess of 33 1/3% of its gross assets.
For purposes of this restriction, collateral arrangements with respect to
any type of swap, option, Futures Contracts and Forward Contracts and
payments of initial and variation margin in connection therewith, are not
considered a pledge of assets.
(3) Invest for the purpose of exercising control or management."
The date of this Supplement is November 1, 1999.
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